.git as file pointing to directory?

Sorry if this is obvious, I can't figure out what term to search for.

At gittogether there was some talk about having .git be a file, not a
folder, with contents pointing to the real .git directory.  Similar to
a symlink, but supported in Windows.  Is there a specific name for
this feature?  Where can I find more details?  Which version of git
introduced this?

Thanks!
Brad