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[[def_alternate_object_database]]alternate object database::
        Via the alternates mechanism, a <<def_repository,repository>>
        can inherit part of its <<def_object_database,object database>>
        from another object database, which is called an "alternate".

[[def_bare_repository]]bare repository::
        A bare repository is normally an appropriately
        named <<def_directory,directory>> with a `.git` suffix that does not
        have a locally checked-out copy of any of the files under
        revision control. That is, all of the Git
        administrative and control files that would normally be present in the
        hidden `.git` sub-directory are directly present in the
        `repository.git` directory instead,
        and no other files are present and checked out. Usually publishers of
        public repositories make bare repositories available.

[[def_blob_object]]blob object::
        Untyped <<def_object,object>>, e.g. the contents of a file.

[[def_branch]]branch::
        A "branch" is a line of development.  The most recent
        <<def_commit,commit>> on a branch is referred to as the tip of
        that branch.  The tip of the branch is <<def_ref,referenced>> by a branch
        <<def_head,head>>, which moves forward as additional development
        is done on the branch.  A single Git
        <<def_repository,repository>> can track an arbitrary number of
        branches, but your <<def_working_tree,working tree>> is
        associated with just one of them (the "current" or "checked out"
        branch), and <<def_HEAD,HEAD>> points to that branch.

[[def_cache]]cache::
        Obsolete for: <<def_index,index>>.

[[def_chain]]chain::
        A list of objects, where each <<def_object,object>> in the list contains
        a reference to its successor (for example, the successor of a
        <<def_commit,commit>> could be one of its <<def_parent,parents>>).

[[def_changeset]]changeset::
        BitKeeper/cvsps speak for "<<def_commit,commit>>". Since Git does not
        store changes, but states, it really does not make sense to use the term
        "changesets" with Git.

[[def_checkout]]checkout::
        The action of updating all or part of the
        <<def_working_tree,working tree>> with a <<def_tree_object,tree object>>
        or <<def_blob_object,blob>> from the
        <<def_object_database,object database>>, and updating the
        <<def_index,index>> and <<def_HEAD,HEAD>> if the whole working tree has
        been pointed at a new <<def_branch,branch>>.

[[def_cherry-picking]]cherry-picking::
        In <<def_SCM,SCM>> jargon, "cherry pick" means to choose a subset of
        changes out of a series of changes (typically commits) and record them
        as a new series of changes on top of a different codebase. In Git, this is
        performed by the "git cherry-pick" command to extract the change introduced
        by an existing <<def_commit,commit>> and to record it based on the tip
        of the current <<def_branch,branch>> as a new commit.

[[def_clean]]clean::
        A <<def_working_tree,working tree>> is clean, if it
        corresponds to the <<def_revision,revision>> referenced by the current
        <<def_head,head>>. Also see "<<def_dirty,dirty>>".

[[def_commit]]commit::
        As a noun: A single point in the
        Git history; the entire history of a project is represented as a
        set of interrelated commits.  The word "commit" is often
        used by Git in the same places other revision control systems
        use the words "revision" or "version".  Also used as a short
        hand for <<def_commit_object,commit object>>.
+
As a verb: The action of storing a new snapshot of the project's
state in the Git history, by creating a new commit representing the current
state of the <<def_index,index>> and advancing <<def_HEAD,HEAD>>
to point at the new commit.

[[def_commit_graph_general]]commit graph concept, representations and usage::
        A synonym for the <<def_DAG,DAG>> structure formed by the commits
        in the object database, <<def_ref,referenced>> by branch tips,
        using their <<def_chain,chain>> of linked commits.
        This structure is the definitive commit graph. The
        graph can be represented in other ways, e.g. the
        <<def_commit_graph_file,"commit-graph" file>>.

[[def_commit_graph_file]]commit-graph file::
        The "commit-graph" (normally hyphenated) file is a supplemental
        representation of the <<def_commit_graph_general,commit graph>>
        which accelerates commit graph walks. The "commit-graph" file is
        stored either in the .git/objects/info directory or in the info
        directory of an alternate object database.

[[def_commit_object]]commit object::
        An <<def_object,object>> which contains the information about a
        particular <<def_revision,revision>>, such as <<def_parent,parents>>, committer,
        author, date and the <<def_tree_object,tree object>> which corresponds
        to the top <<def_directory,directory>> of the stored
        revision.

[[def_commit-ish]]commit-ish (also committish)::
        A <<def_commit_object,commit object>> or an
        <<def_object,object>> that can be recursively dereferenced to
        a commit object.
        The following are all commit-ishes:
        a commit object,
        a <<def_tag_object,tag object>> that points to a commit
        object,
        a tag object that points to a tag object that points to a
        commit object,
        etc.

[[def_core_git]]core Git::
        Fundamental data structures and utilities of Git. Exposes only limited
        source code management tools.

[[def_DAG]]DAG::
        Directed acyclic graph. The <<def_commit_object,commit objects>> form a
        directed acyclic graph, because they have parents (directed), and the
        graph of commit objects is acyclic (there is no <<def_chain,chain>>
        which begins and ends with the same <<def_object,object>>).

[[def_dangling_object]]dangling object::
        An <<def_unreachable_object,unreachable object>> which is not
        <<def_reachable,reachable>> even from other unreachable objects; a
        dangling object has no references to it from any
        reference or <<def_object,object>> in the <<def_repository,repository>>.

[[def_detached_HEAD]]detached HEAD::
        Normally the <<def_HEAD,HEAD>> stores the name of a
        <<def_branch,branch>>, and commands that operate on the
        history HEAD represents operate on the history leading to the
        tip of the branch the HEAD points at.  However, Git also
        allows you to <<def_checkout,check out>> an arbitrary
        <<def_commit,commit>> that isn't necessarily the tip of any
        particular branch.  The HEAD in such a state is called
        "detached".
+
Note that commands that operate on the history of the current branch
(e.g. `git commit` to build a new history on top of it) still work
while the HEAD is detached. They update the HEAD to point at the tip
of the updated history without affecting any branch.  Commands that
update or inquire information _about_ the current branch (e.g. `git
branch --set-upstream-to` that sets what remote-tracking branch the
current branch integrates with) obviously do not work, as there is no
(real) current branch to ask about in this state.

[[def_directory]]directory::
        The list you get with "ls" :-)

[[def_dirty]]dirty::
        A <<def_working_tree,working tree>> is said to be "dirty" if
        it contains modifications which have not been <<def_commit,committed>> to the current
        <<def_branch,branch>>.

[[def_evil_merge]]evil merge::
        An evil merge is a <<def_merge,merge>> that introduces changes that
        do not appear in any <<def_parent,parent>>.

[[def_fast_forward]]fast-forward::
        A fast-forward is a special type of <<def_merge,merge>> where you have a
        <<def_revision,revision>> and you are "merging" another
        <<def_branch,branch>>'s changes that happen to be a descendant of what
        you have. In such a case, you do not make a new <<def_merge,merge>>
        <<def_commit,commit>> but instead just update your branch to point at the same
        revision as the branch you are merging. This will happen frequently on a
        <<def_remote_tracking_branch,remote-tracking branch>> of a remote
        <<def_repository,repository>>.

[[def_fetch]]fetch::
        Fetching a <<def_branch,branch>> means to get the
        branch's <<def_head_ref,head ref>> from a remote
        <<def_repository,repository>>, to find out which objects are
        missing from the local <<def_object_database,object database>>,
        and to get them, too.  See also linkgit:git-fetch[1].

[[def_file_system]]file system::
        Linus Torvalds originally designed Git to be a user space file system,
        i.e. the infrastructure to hold files and directories. That ensured the
        efficiency and speed of Git.

[[def_git_archive]]Git archive::
        Synonym for <<def_repository,repository>> (for arch people).

[[def_gitfile]]gitfile::
        A plain file `.git` at the root of a working tree that
        points at the directory that is the real repository.

[[def_grafts]]grafts::
        Grafts enables two otherwise different lines of development to be joined
        together by recording fake ancestry information for commits. This way
        you can make Git pretend the set of <<def_parent,parents>> a <<def_commit,commit>> has
        is different from what was recorded when the commit was
        created. Configured via the `.git/info/grafts` file.
+
Note that the grafts mechanism is outdated and can lead to problems
transferring objects between repositories; see linkgit:git-replace[1]
for a more flexible and robust system to do the same thing.

[[def_hash]]hash::
        In Git's context, synonym for <<def_object_name,object name>>.

[[def_head]]head::
        A <<def_ref,named reference>> to the <<def_commit,commit>> at the tip of a
        <<def_branch,branch>>.  Heads are stored in a file in
        `$GIT_DIR/refs/heads/` directory, except when using packed refs. (See
        linkgit:git-pack-refs[1].)

[[def_HEAD]]HEAD::
        The current <<def_branch,branch>>.  In more detail: Your <<def_working_tree,
        working tree>> is normally derived from the state of the tree
        referred to by HEAD.  HEAD is a reference to one of the
        <<def_head,heads>> in your repository, except when using a
        <<def_detached_HEAD,detached HEAD>>, in which case it directly
        references an arbitrary commit.

[[def_head_ref]]head ref::
        A synonym for <<def_head,head>>.

[[def_hook]]hook::
        During the normal execution of several Git commands, call-outs are made
        to optional scripts that allow a developer to add functionality or
        checking. Typically, the hooks allow for a command to be pre-verified
        and potentially aborted, and allow for a post-notification after the
        operation is done. The hook scripts are found in the
        `$GIT_DIR/hooks/` directory, and are enabled by simply
        removing the `.sample` suffix from the filename. In earlier versions
        of Git you had to make them executable.

[[def_index]]index::
        A collection of files with stat information, whose contents are stored
        as objects. The index is a stored version of your
        <<def_working_tree,working tree>>. Truth be told, it can also contain a second, and even
        a third version of a working tree, which are used
        when <<def_merge,merging>>.

[[def_index_entry]]index entry::
        The information regarding a particular file, stored in the
        <<def_index,index>>. An index entry can be unmerged, if a
        <<def_merge,merge>> was started, but not yet finished (i.e. if
        the index contains multiple versions of that file).

[[def_master]]master::
        The default development <<def_branch,branch>>. Whenever you
        create a Git <<def_repository,repository>>, a branch named
        "master" is created, and becomes the active branch. In most
        cases, this contains the local development, though that is
        purely by convention and is not required.

[[def_merge]]merge::
        As a verb: To bring the contents of another
        <<def_branch,branch>> (possibly from an external
        <<def_repository,repository>>) into the current branch.  In the
        case where the merged-in branch is from a different repository,
        this is done by first <<def_fetch,fetching>> the remote branch
        and then merging the result into the current branch.  This
        combination of fetch and merge operations is called a
        <<def_pull,pull>>.  Merging is performed by an automatic process
        that identifies changes made since the branches diverged, and
        then applies all those changes together.  In cases where changes
        conflict, manual intervention may be required to complete the
        merge.
+
As a noun: unless it is a <<def_fast_forward,fast-forward>>, a
successful merge results in the creation of a new <<def_commit,commit>>
representing the result of the merge, and having as
<<def_parent,parents>> the tips of the merged <<def_branch,branches>>.
This commit is referred to as a "merge commit", or sometimes just a
"merge".

[[def_object]]object::
        The unit of storage in Git. It is uniquely identified by the
        <<def_SHA1,SHA-1>> of its contents. Consequently, an
        object cannot be changed.

[[def_object_database]]object database::
        Stores a set of "objects", and an individual <<def_object,object>> is
        identified by its <<def_object_name,object name>>. The objects usually
        live in `$GIT_DIR/objects/`.

[[def_object_identifier]]object identifier (oid)::
        Synonym for <<def_object_name,object name>>.

[[def_object_name]]object name::
        The unique identifier of an <<def_object,object>>.  The
        object name is usually represented by a 40 character
        hexadecimal string.  Also colloquially called <<def_SHA1,SHA-1>>.

[[def_object_type]]object type::
        One of the identifiers "<<def_commit_object,commit>>",
        "<<def_tree_object,tree>>", "<<def_tag_object,tag>>" or
        "<<def_blob_object,blob>>" describing the type of an
        <<def_object,object>>.

[[def_octopus]]octopus::
        To <<def_merge,merge>> more than two <<def_branch,branches>>.

[[def_origin]]origin::
        The default upstream <<def_repository,repository>>. Most projects have
        at least one upstream project which they track. By default
        'origin' is used for that purpose. New upstream updates
        will be fetched into <<def_remote_tracking_branch,remote-tracking branches>> named
        origin/name-of-upstream-branch, which you can see using
        `git branch -r`.

[[def_overlay]]overlay::
        Only update and add files to the working directory, but don't
        delete them, similar to how 'cp -R' would update the contents
        in the destination directory.  This is the default mode in a
        <<def_checkout,checkout>> when checking out files from the
        <<def_index,index>> or a <<def_tree-ish,tree-ish>>.  In
        contrast, no-overlay mode also deletes tracked files not
        present in the source, similar to 'rsync --delete'.

[[def_pack]]pack::
        A set of objects which have been compressed into one file (to save space
        or to transmit them efficiently).

[[def_pack_index]]pack index::
        The list of identifiers, and other information, of the objects in a
        <<def_pack,pack>>, to assist in efficiently accessing the contents of a
        pack.

[[def_pathspec]]pathspec::
        Pattern used to limit paths in Git commands.
+
Pathspecs are used on the command line of "git ls-files", "git
ls-tree", "git add", "git grep", "git diff", "git checkout",
and many other commands to
limit the scope of operations to some subset of the tree or
working tree.  See the documentation of each command for whether
paths are relative to the current directory or toplevel.  The
pathspec syntax is as follows:
+
--

* any path matches itself
* the pathspec up to the last slash represents a
  directory prefix.  The scope of that pathspec is
  limited to that subtree.
* the rest of the pathspec is a pattern for the remainder
  of the pathname.  Paths relative to the directory
  prefix will be matched against that pattern using fnmatch(3);
  in particular, '*' and '?' _can_ match directory separators.

--
+
For example, Documentation/*.jpg will match all .jpg files
in the Documentation subtree,
including Documentation/chapter_1/figure_1.jpg.
+
A pathspec that begins with a colon `:` has special meaning.  In the
short form, the leading colon `:` is followed by zero or more "magic
signature" letters (which optionally is terminated by another colon `:`),
and the remainder is the pattern to match against the path.
The "magic signature" consists of ASCII symbols that are neither
alphanumeric, glob, regex special characters nor colon.
The optional colon that terminates the "magic signature" can be
omitted if the pattern begins with a character that does not belong to
"magic signature" symbol set and is not a colon.
+
In the long form, the leading colon `:` is followed by an open
parenthesis `(`, a comma-separated list of zero or more "magic words",
and a close parentheses `)`, and the remainder is the pattern to match
against the path.
+
A pathspec with only a colon means "there is no pathspec". This form
should not be combined with other pathspec.
+
--
top;;
        The magic word `top` (magic signature: `/`) makes the pattern
        match from the root of the working tree, even when you are
        running the command from inside a subdirectory.

literal;;
        Wildcards in the pattern such as `*` or `?` are treated
        as literal characters.

icase;;
        Case insensitive match.

glob;;
        Git treats the pattern as a shell glob suitable for
        consumption by fnmatch(3) with the FNM_PATHNAME flag:
        wildcards in the pattern will not match a / in the pathname.
        For example, "Documentation/{asterisk}.html" matches
        "Documentation/git.html" but not "Documentation/ppc/ppc.html"
        or "tools/perf/Documentation/perf.html".
+
Two consecutive asterisks ("`**`") in patterns matched against
full pathname may have special meaning:

 - A leading "`**`" followed by a slash means match in all
   directories. For example, "`**/foo`" matches file or directory
   "`foo`" anywhere, the same as pattern "`foo`". "`**/foo/bar`"
   matches file or directory "`bar`" anywhere that is directly
   under directory "`foo`".

 - A trailing "`/**`" matches everything inside. For example,
   "`abc/**`" matches all files inside directory "abc", relative
   to the location of the `.gitignore` file, with infinite depth.

 - A slash followed by two consecutive asterisks then a slash
   matches zero or more directories. For example, "`a/**/b`"
   matches "`a/b`", "`a/x/b`", "`a/x/y/b`" and so on.

 - Other consecutive asterisks are considered invalid.
+
Glob magic is incompatible with literal magic.

attr;;
After `attr:` comes a space separated list of "attribute
requirements", all of which must be met in order for the
path to be considered a match; this is in addition to the
usual non-magic pathspec pattern matching.
See linkgit:gitattributes[5].
+
Each of the attribute requirements for the path takes one of
these forms:

- "`ATTR`" requires that the attribute `ATTR` be set.

- "`-ATTR`" requires that the attribute `ATTR` be unset.

- "`ATTR=VALUE`" requires that the attribute `ATTR` be
  set to the string `VALUE`.

- "`!ATTR`" requires that the attribute `ATTR` be
  unspecified.
+
Note that when matching against a tree object, attributes are still
obtained from working tree, not from the given tree object.

exclude;;
        After a path matches any non-exclude pathspec, it will be run
        through all exclude pathspecs (magic signature: `!` or its
        synonym `^`). If it matches, the path is ignored.  When there
        is no non-exclude pathspec, the exclusion is applied to the
        result set as if invoked without any pathspec.
--

[[def_parent]]parent::
        A <<def_commit_object,commit object>> contains a (possibly empty) list
        of the logical predecessor(s) in the line of development, i.e. its
        parents.

[[def_pickaxe]]pickaxe::
        The term <<def_pickaxe,pickaxe>> refers to an option to the diffcore
        routines that help select changes that add or delete a given text
        string. With the `--pickaxe-all` option, it can be used to view the full
        <<def_changeset,changeset>> that introduced or removed, say, a
        particular line of text. See linkgit:git-diff[1].

[[def_plumbing]]plumbing::
        Cute name for <<def_core_git,core Git>>.

[[def_porcelain]]porcelain::
        Cute name for programs and program suites depending on
        <<def_core_git,core Git>>, presenting a high level access to
        core Git. Porcelains expose more of a <<def_SCM,SCM>>
        interface than the <<def_plumbing,plumbing>>.

[[def_per_worktree_ref]]per-worktree ref::
        Refs that are per-<<def_worktree,worktree>>, rather than
        global.  This is presently only <<def_HEAD,HEAD>> and any refs
        that start with `refs/bisect/`, but might later include other
        unusual refs.

[[def_pseudoref]]pseudoref::
        Pseudorefs are a class of files under `$GIT_DIR` which behave
        like refs for the purposes of rev-parse, but which are treated
        specially by git.  Pseudorefs both have names that are all-caps,
        and always start with a line consisting of a
        <<def_SHA1,SHA-1>> followed by whitespace.  So, HEAD is not a
        pseudoref, because it is sometimes a symbolic ref.  They might
        optionally contain some additional data.  `MERGE_HEAD` and
        `CHERRY_PICK_HEAD` are examples.  Unlike
        <<def_per_worktree_ref,per-worktree refs>>, these files cannot
        be symbolic refs, and never have reflogs.  They also cannot be
        updated through the normal ref update machinery.  Instead,
        they are updated by directly writing to the files.  However,
        they can be read as if they were refs, so `git rev-parse
        MERGE_HEAD` will work.

[[def_pull]]pull::
        Pulling a <<def_branch,branch>> means to <<def_fetch,fetch>> it and
        <<def_merge,merge>> it.  See also linkgit:git-pull[1].

[[def_push]]push::
        Pushing a <<def_branch,branch>> means to get the branch's
        <<def_head_ref,head ref>> from a remote <<def_repository,repository>>,
        find out if it is an ancestor to the branch's local
        head ref, and in that case, putting all
        objects, which are <<def_reachable,reachable>> from the local
        head ref, and which are missing from the remote
        repository, into the remote
        <<def_object_database,object database>>, and updating the remote
        head ref. If the remote <<def_head,head>> is not an
        ancestor to the local head, the push fails.

[[def_reachable]]reachable::
        All of the ancestors of a given <<def_commit,commit>> are said to be
        "reachable" from that commit. More
        generally, one <<def_object,object>> is reachable from
        another if we can reach the one from the other by a <<def_chain,chain>>
        that follows <<def_tag,tags>> to whatever they tag,
        <<def_commit_object,commits>> to their parents or trees, and
        <<def_tree_object,trees>> to the trees or <<def_blob_object,blobs>>
        that they contain.

[[def_reachability_bitmap]]reachability bitmaps::
        Reachability bitmaps store information about the
        <<def_reachable,reachability>> of a selected set of commits in
        a packfile, or a multi-pack index (MIDX), to speed up object search.
        The bitmaps are stored in a ".bitmap" file. A repository may have at
        most one bitmap file in use. The bitmap file may belong to either one
        pack, or the repository's multi-pack index (if it exists).

[[def_rebase]]rebase::
        To reapply a series of changes from a <<def_branch,branch>> to a
        different base, and reset the <<def_head,head>> of that branch
        to the result.

[[def_ref]]ref::
        A name that begins with `refs/` (e.g. `refs/heads/master`)
        that points to an <<def_object_name,object name>> or another
        ref (the latter is called a <<def_symref,symbolic ref>>).
        For convenience, a ref can sometimes be abbreviated when used
        as an argument to a Git command; see linkgit:gitrevisions[7]
        for details.
        Refs are stored in the <<def_repository,repository>>.
+
The ref namespace is hierarchical.
Different subhierarchies are used for different purposes (e.g. the
`refs/heads/` hierarchy is used to represent local branches).
+
There are a few special-purpose refs that do not begin with `refs/`.
The most notable example is `HEAD`.

[[def_reflog]]reflog::
        A reflog shows the local "history" of a ref.  In other words,
        it can tell you what the 3rd last revision in _this_ repository
        was, and what was the current state in _this_ repository,
        yesterday 9:14pm.  See linkgit:git-reflog[1] for details.

[[def_refspec]]refspec::
        A "refspec" is used by <<def_fetch,fetch>> and
        <<def_push,push>> to describe the mapping between remote
        <<def_ref,ref>> and local ref.

[[def_remote]]remote repository::
        A <<def_repository,repository>> which is used to track the same
        project but resides somewhere else. To communicate with remotes,
        see <<def_fetch,fetch>> or <<def_push,push>>.

[[def_remote_tracking_branch]]remote-tracking branch::
        A <<def_ref,ref>> that is used to follow changes from another
        <<def_repository,repository>>. It typically looks like
        'refs/remotes/foo/bar' (indicating that it tracks a branch named
        'bar' in a remote named 'foo'), and matches the right-hand-side of
        a configured fetch <<def_refspec,refspec>>. A remote-tracking
        branch should not contain direct modifications or have local
        commits made to it.

[[def_repository]]repository::
        A collection of <<def_ref,refs>> together with an
        <<def_object_database,object database>> containing all objects
        which are <<def_reachable,reachable>> from the refs, possibly
        accompanied by meta data from one or more <<def_porcelain,porcelains>>. A
        repository can share an object database with other repositories
        via <<def_alternate_object_database,alternates mechanism>>.

[[def_resolve]]resolve::
        The action of fixing up manually what a failed automatic
        <<def_merge,merge>> left behind.

[[def_revision]]revision::
        Synonym for <<def_commit,commit>> (the noun).

[[def_rewind]]rewind::
        To throw away part of the development, i.e. to assign the
        <<def_head,head>> to an earlier <<def_revision,revision>>.

[[def_SCM]]SCM::
        Source code management (tool).

[[def_SHA1]]SHA-1::
        "Secure Hash Algorithm 1"; a cryptographic hash function.
        In the context of Git used as a synonym for <<def_object_name,object name>>.

[[def_shallow_clone]]shallow clone::
        Mostly a synonym to <<def_shallow_repository,shallow repository>>
        but the phrase makes it more explicit that it was created by
        running `git clone --depth=...` command.

[[def_shallow_repository]]shallow repository::
        A shallow <<def_repository,repository>> has an incomplete
        history some of whose <<def_commit,commits>> have <<def_parent,parents>> cauterized away (in other
        words, Git is told to pretend that these commits do not have the
        parents, even though they are recorded in the <<def_commit_object,commit
        object>>). This is sometimes useful when you are interested only in the
        recent history of a project even though the real history recorded in the
        upstream is much larger. A shallow repository
        is created by giving the `--depth` option to linkgit:git-clone[1], and
        its history can be later deepened with linkgit:git-fetch[1].

[[def_stash]]stash entry::
        An <<def_object,object>> used to temporarily store the contents of a
        <<def_dirty,dirty>> working directory and the index for future reuse.

[[def_submodule]]submodule::
        A <<def_repository,repository>> that holds the history of a
        separate project inside another repository (the latter of
        which is called <<def_superproject, superproject>>).

[[def_superproject]]superproject::
        A <<def_repository,repository>> that references repositories
        of other projects in its working tree as <<def_submodule,submodules>>.
        The superproject knows about the names of (but does not hold
        copies of) commit objects of the contained submodules.

[[def_symref]]symref::
        Symbolic reference: instead of containing the <<def_SHA1,SHA-1>>
        id itself, it is of the format 'ref: refs/some/thing' and when
        referenced, it recursively dereferences to this reference.
        '<<def_HEAD,HEAD>>' is a prime example of a symref. Symbolic
        references are manipulated with the linkgit:git-symbolic-ref[1]
        command.

[[def_tag]]tag::
        A <<def_ref,ref>> under `refs/tags/` namespace that points to an
        object of an arbitrary type (typically a tag points to either a
        <<def_tag_object,tag>> or a <<def_commit_object,commit object>>).
        In contrast to a <<def_head,head>>, a tag is not updated by
        the `commit` command. A Git tag has nothing to do with a Lisp
        tag (which would be called an <<def_object_type,object type>>
        in Git's context). A tag is most typically used to mark a particular
        point in the commit ancestry <<def_chain,chain>>.

[[def_tag_object]]tag object::
        An <<def_object,object>> containing a <<def_ref,ref>> pointing to
        another object, which can contain a message just like a
        <<def_commit_object,commit object>>. It can also contain a (PGP)
        signature, in which case it is called a "signed tag object".

[[def_topic_branch]]topic branch::
        A regular Git <<def_branch,branch>> that is used by a developer to
        identify a conceptual line of development. Since branches are very easy
        and inexpensive, it is often desirable to have several small branches
        that each contain very well defined concepts or small incremental yet
        related changes.

[[def_tree]]tree::
        Either a <<def_working_tree,working tree>>, or a <<def_tree_object,tree
        object>> together with the dependent <<def_blob_object,blob>> and tree objects
        (i.e. a stored representation of a working tree).

[[def_tree_object]]tree object::
        An <<def_object,object>> containing a list of file names and modes along
        with refs to the associated blob and/or tree objects. A
        <<def_tree,tree>> is equivalent to a <<def_directory,directory>>.

[[def_tree-ish]]tree-ish (also treeish)::
        A <<def_tree_object,tree object>> or an <<def_object,object>>
        that can be recursively dereferenced to a tree object.
        Dereferencing a <<def_commit_object,commit object>> yields the
        tree object corresponding to the <<def_revision,revision>>'s
        top <<def_directory,directory>>.
        The following are all tree-ishes:
        a <<def_commit-ish,commit-ish>>,
        a tree object,
        a <<def_tag_object,tag object>> that points to a tree object,
        a tag object that points to a tag object that points to a tree
        object,
        etc.

[[def_unmerged_index]]unmerged index::
        An <<def_index,index>> which contains unmerged
        <<def_index_entry,index entries>>.

[[def_unreachable_object]]unreachable object::
        An <<def_object,object>> which is not <<def_reachable,reachable>> from a
        <<def_branch,branch>>, <<def_tag,tag>>, or any other reference.

[[def_upstream_branch]]upstream branch::
        The default <<def_branch,branch>> that is merged into the branch in
        question (or the branch in question is rebased onto). It is configured
        via branch.<name>.remote and branch.<name>.merge. If the upstream branch
        of 'A' is 'origin/B' sometimes we say "'A' is tracking 'origin/B'".

[[def_working_tree]]working tree::
        The tree of actual checked out files.  The working tree normally
        contains the contents of the <<def_HEAD,HEAD>> commit's tree,
        plus any local changes that you have made but not yet committed.

[[def_worktree]]worktree::
        A repository can have zero (i.e. bare repository) or one or
        more worktrees attached to it. One "worktree" consists of a
        "working tree" and repository metadata, most of which are
        shared among other worktrees of a single repository, and
        some of which are maintained separately per worktree
        (e.g. the index, HEAD and pseudorefs like MERGE_HEAD,
        per-worktree refs and per-worktree configuration file).