You can find the definition here: Beginner's Guide to Glossary of Terms on Straight Dope Message Boards
Zombie – A zombie is an old thread that had ceased activity for some time (was dead), but has now been revived by a new post (was brought back to life like a zombie).
As far as the rules go, pointless revivals of zombies should be avoided, but if you have something new or interesting to add to the conversation or you have an update to something that was posted that’s usually not a problem.
This is more strictly enforced in some forums (like GQ) and less strictly enforced in others (like MPSIMS and IMHO).