Posting in a thread that hasn't been posted in for a long time, bumping them to the top instead of letting them fade away in peace. It's worst when it's done by someone just saying "i agree" or something equally useless, but technically okay when you've got something meaningful to add to the old discussion.
So called because old threads are "dead" threads, and so posting in them is raising the dead; necromancy. Or threadcromancy.