trump card

Từ điển Oxford Advanced Learner 8th

ˈtrump card [trump card] BrE NAmE noun
1. = ↑trump (¹)
2. 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
•He was able to get what he wanted without having to play his trump card.

Example Bank:
•Many private schools use small classes as their trump card in marketing campaigns.