HEAD and ORIG_HEAD in Git

HEAD is (direct or indirect, i.e. symbolic) reference to the current commit. It is a commit that you have checked in the working directory (unless you made some changes, or equivalent), and it is a commit on top of which "git commit" would make a new one. Usually HEAD is symbolic reference to some other named branch; this branch is currently checked out branch, or current branch. HEAD can also point directly to a commit; this state is called "detached HEAD", and can be understood as being on unnamed, anonymous branch.

And @ alone is a shortcut for HEAD, since Git 1.8.5

ORIG_HEAD is previous state of HEAD, set by commands that have possibly dangerous behavior, to be easy to revert them. It is less useful now that Git has reflog: HEAD@{1} is roughly equivalent to ORIG_HEAD (HEAD@{1} is always last value of HEAD, ORIG_HEAD is last value of HEAD before dangerous operation).

For more information read git(1) manpage / [gitrevisions(7) manpage][git-revisions], Git User's Manual, the Git Community Book and Git Glossary

From git reset

"pull" or "merge" always leaves the original tip of the current branch in ORIG_HEAD.

git reset --hard ORIG_HEAD

Resetting hard to it brings your index file and the working tree back to that state, and resets the tip of the branch to that commit.

git reset --merge ORIG_HEAD

After inspecting the result of the merge, you may find that the change in the other branch is unsatisfactory. Running "git reset --hard ORIG_HEAD" will let you go back to where you were, but it will discard your local changes, which you do not want. "git reset --merge" keeps your local changes.

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Before any patches are applied, ORIG_HEAD is set to the tip of the current branch.
This is useful if you have problems with multiple commits, like running 'git am' on the wrong branch or an error in the commits that is more easily fixed by changing the mailbox (e.g. +errors in the "From:" lines).

In addition, merge always sets '.git/ORIG_HEAD' to the original state of HEAD so a problematic merge can be removed by using 'git reset ORIG_HEAD'.

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Note: from here

HEAD is a moving pointer. Sometimes it means the current branch, sometimes it doesn't.

So HEAD is NOT a synonym for "current branch" everywhere already.

HEAD means "current" everywhere in git, but it does not necessarily mean "current branch" (i.e. detached HEAD).

But it almost always means the "current commit".
It is the commit "git commit" builds on top of, and "git diff --cached" and "git status" compare against.
It means the current branch only in very limited contexts (exactly when we want a branch name to operate on --- resetting and growing the branch tip via commit/rebase/etc.).

Reflog is a vehicle to go back in time and time machines have interesting interaction with the notion of "current".

HEAD@{5.minutes.ago} could mean "dereference HEAD symref to find out what branch we are on RIGHT NOW, and then find out where the tip of that branch was 5 minutes ago".
Alternatively it could mean "what is the commit I would have referred to as HEAD 5 minutes ago, e.g. if I did "git show HEAD" back then".

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git1.8.4 (July 2013) introduces introduced a new notation!
(Actually, it will be for 1.8.5, Q4 2013: reintroduced with commit 9ba89f4), by Felipe Contreras.

Instead of typing four capital letters "HEAD", you can say "@" now,
e.g. "git log @".

See commit cdfd948

Typing 'HEAD' is tedious, especially when we can use '@' instead.

The reason for choosing '@' is that it follows naturally from the ref@op syntax (e.g. HEAD@{u}), except we have no ref, and no operation, and when we don't have those, it makes sens to assume 'HEAD'.

So now we can use 'git show @~1', and all that goody goodness.

Until now '@' was a valid name, but it conflicts with this idea, so let's make it invalid. Probably very few people, if any, used this name.