Tennis term for a point on which the receiving (as opposed to serving) player or doubles team can win a game. Break points are notable because in tennis, the server usually has the advantage, so having the chance to "break serve" (win a game while receiving) is important.

For example's sake, let's say Jennifer Capriati is serving to Venus Williams. The game score is 30-40, meaning that the receiver (Williams) has 40, and if she wins the next point, she wins the game (and breaks Capriati's serve). Thus, she has a break point.
Similarly, if the score was 15-40, Williams could win either of the next 2 points and win the game, so she'd have a double break point. 0-40 would be triple break point.

It's important to note that the break point occurs before the point is actually played. In other words, Williams doesn't need to win the point for it to be called a break point. So the break point is the OPPORTUNITY to break serve by winning the upcoming point, not the actual winning of the point.

Similarly, set point is the chance to win a set and match point is the chance to win a match by winning the next point.

Although tennis isn't a very stats-intensive sport (unlike, say, baseball), break point conversion percentage in a tournament is often tracked.