git fsck: unreachable vs. dangling

Hi,

reading through the fsck docs [1] I'm having a hard time understanding what the difference between "unreachable" and "dangling" objects are.

By example, suppose I have a commit A that is the tip of exactly one branch (and no tag or other ref points to A). If I delete that branch, is A now dangling, or unreachable, or both?

[1] http://git-scm.com/docs/git-fsck.html