From DominionStrategy Wiki
Cards are referred to as dead if they are drawn at a time when they can't be used, or can't be used effectively. Some cards, like Curses and the majority of Victory cards, are always or almost always dead; they have little or no function during gameplay other than to fill your hand with unplayable cards. However, cards that are usually playable may be described as "dead" under some circumstances as well:
- The most common example of drawing cards "dead" is when you play a terminal draw card like Smithy and draw Actions which you can't play because you have no actions remaining.
- Throne Room variants can be drawn with no actions to use as targets.
- Treasure Map drawn without a second Treasure Map in hand can be a dead card.
- Sea Hag becomes mostly a dead card once the Curse pile is empty.
- Early-game trashers become dead cards once you have trashed all of the cards you want to trash.
- Baron may be considered dead if it is drawn without an Estate in hand by a player who does not want to gain an Estate.
A related phenomenon is the terminal collision, when you have two (or more) terminal Actions in hand but can only play one of them—in effect, you must choose one of the two cards to be dead for this turn.