A convention whereby a bid of 4♣ asks about the number of aces held by partner. In response:
4♦ shows zero or four aces
4♥ shows one ace
4♠ shows two aces
4NT shows three aces
Subsequently, 5♣ enquires about kings with corresponding responses.
Due to the usefulness of a 4♣ bid for other meanings, such as a cue bid, a splinter or as a natural bid, it is common to play that 4♣ is only Gerber in very limited situations, such as a direct response to a 1NT or 2NT opening. Gerber is one of the most abused conventions ever invented.