A mercy rule, slaughter rule, knockout rule, or skunk rule ends a two-competitor sports competition earlier than the scheduled endpoint if one competitor has a very large and presumably insurmountable scoring lead over the other. It is called the mercy rule because it spares further humiliation for the loser. It is common in youth sports in North America, where running up the score is considered unsporting. It is especially common in baseball and softball in which there is no game clock and a dominant team could in theory continue an inning endlessly.
The rules vary widely, depending on the level of competition, but nearly all youth sports leagues and high school sports associations and many college sports associations in the United States have mercy rules for sports including baseball, softball, American football and association football.
However, mercy rules usually do not take effect until a prescribed point in the game (like the second half of an association football game). Thus, one team, particularly if it is decidedly better than a weaker opponent, can still "run up the score" before the rule takes effect. For instance, in American football, one team could be ahead by 70 points with three minutes left in the first half; in baseball, the better team could have a 20-run lead in the second inning, but the game would still continue.