How would you define it?
In poker, it does not necessarily mean dead literally. Dead money simply means the amount of money placed in the pot other than the equal amounts bet by the remaining active players.
For those who have folded, the money they have contributed is considered to be dead money. On the other hand, there’s also a thing like a dead blind which is a blind posted by a player returning to a game after missing the blinds or an odd chip left in the pot from a previous deal.