The kal-if-fee is a Vulcan challenge in which two male combatants fight for the right to mate with a certain female. During the Vulcan mating ritual known as koon-ut-kal-if-fee, a female can claim kal-if-fee if she doesn't wish to be with the male that was betrothed to her in childhood. Once the female has declared kal-if-fee, the male that was arranged for her must battle her chosen mate.
In 2267, T'Pring declared the kal-if-fee against Spock, and he was forced to fight Captain James T. Kirk to death, when T'Pring chose Kirk as her champion over Stonn, her chosen mate. (TOS episode: "Amok Time"; novel: Crucible: The Fire and the Rose).