(in some card games) a card that belongs to the suit (= one of the four sets in a pack of cards) that has been chosen for a particular game to have a higher value than the other three suits
something that gives you an advantage over other people, especially when they do not know what it is and you are able to use it to surprise them
Example:
- He was able to get what he wanted without having to play his trump card.
- Many private schools use small classes as their trump card in marketing campaigns.