Break

The loss of a game by the server. In better-grade tennis, having the serve is extremely important, and a set often goes along with first one player winning when he serves, and then the other player winning the next game when he serves. So when a server loses a game, it means that if his opponent merely holds his serve in the remaining games of the set, he will win it. In a tie-breaker, losing a point on one's serve is a minibreak, for the same sort of reasoning.