The act of sacrificing a card that would ordinarily be an eventual winner (such as an offside King) to establish an entry into partner's hand.
The Galileo coup is so named because Galileo Galilei is usually credited with the invention of the telescope; this coup arises when the contract is in a suit in which the declaring side is missing both the Ace and King; if successful, the defenders end up being forced to play the Ace and King of trumps to the same trick, thus "telescoping" their two trump tricks into one.
The Merrimac coup is the act of sacrificing an honour (usually a King) in order to remove an entry from an opponent's hand.
The Scissors coup is so named because it cuts communications between defenders, most commonly by discarding a key card from either the declarer's own hand or dummy.
Even when the gambit does not yield a material gain, it usually induces a big psychological impact on the opponents who were offered a trick for free but couldn't have believed it were possible.
Declarer leads through one of the defenders hoping they will play the king from Kx which then falls under their partner's stiff ace.
An Idiot coup can also refer to a play that appears to present an alternative (losing) option to an opponent, but upon closer inspection could not possibly be the right one.