What does the “at” @ sign/symbol/character mean in Git?

Question

The at-sign @ is often used in git to specify revisions in different ways. For example,

1. @{} specifies the r

Answer 1

As of Git version 1.8.5, the at-sign @, without a leading branch/reference name and ordinal {n} suffix like HEAD@{1} and master@{1}, is just a synonym/alias/shortcut for the special Git reference HEAD:

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

So for these commands

git rebase -i @~4
git log @^^..@

you can simply substitute the first occurrence of @ with HEAD (or head if using Windows or OS X)

git rebase -i HEAD~4
git log HEAD^^..HEAD

So what does HEAD mean? As explained by the official Linux Kernel Git documentation for specifying Git revisions, HEAD is a special shortcut reference for the commit that you currently have checked-out as your working copy (or in Git terms, your "working tree"):

HEAD names the commit on which you based the changes in the working tree.

You can also read these other Stack Overflow questions on what the special reference HEAD means:

1. HEAD and ORIG_HEAD in Git.
2. What is git HEAD, exactly?.

VonC also found interesting information about why @ was chosen as a shortcut for head in this Stack Overflow answer (the last section at the bottom).