Ace in the hole is an idiom that is often used to describe a hidden advantage or a secret strategy that can be employed to give someone an edge. There are several synonyms one can use to convey the same meaning, such as trump card, secret weapon, hidden agenda, sleeper, and wildcard. Each of these terms has its own nuances, and they can be used in various contexts to convey a slightly different meaning. For instance, a trump card may refer to a decisive advantage that can win a game, while a sleeper may refer to a player who is underestimated but turns out to be the surprise hero in the end. Regardless of the word used, they all convey the same idea of having something up one's sleeve to gain an advantage.