The deliberate sacrifice of a trick in order to gain two or more other tricks.
South plays in 4♥ after West opened the bidding. West leads the ♣K, which you allow to win. You win the second club and cash a top trump, finding East void. You should continue by cashing the ♦A and leading a low heart. You are giving up a trump trick but, by securing an entry to dummy with the ♥8, you get tricks with the king and queen of diamonds in return.