Trump card -a valuable resource that may be used, especially as a surprise, in order to gain an advantage.

You're most powerful option in succeeding at anything in life. Unfortunately, once used, the trump card can rarely be used on the same person again.

The trump card is the winning card. If you play a trump card during a game of Spades, it means that you take the whole trick — in other words, you'll pick up every card on the table. Any card game in which one card or suit ranks the highest has a trump card, the card that wins a hand.