When you give someone (you trust) your password and they start messing around with it as if it is theirs and reply to your e-mails/PMs usually stating that they're your account ghost.
Lily's friend: Hey, wussup?
Lily: Nothing, I went to the Green Day concert. Then I got an awesome shirt from the concert.
Lily's Friend: Really?! Awesome!
'Account ghost': Lily is not here at the moment so her account ghost shall be filling in for her. :B
Lily's Friend: WTF?! o.0