Foul ball

In baseball, a foul ball is any hit ball that goes in foul territory, which is the area of the field not in between the bases (when looking from home plate to the pitcher's mound), without first going beyond the bases nearest to home plate, or being touched by a player, umpire, or an object not part of the ground while on or over fair territory (which is the part of the field that is not foul territory). 
Each foul ball is a strike against the batter, except when this would result in the batter being struck out. The batting team can not score on a foul ball unless the batter flies out.
Other sports.
Other bat-and-ball games have things that are similar to foul balls. For example, some forms of street cricket, like bete-ombro, penalize the batter if the ball goes behind them, which gives the batter 180 degrees of fair playing area, rather than the 90 degrees of fair playing area in baseball.