In baseball, a foul ball is a batted ball that is not a foul tip, and that...

• settles on foul ground between home and first base, or home and third base, or
• bounds past first or third base on or over foul territory, or
• first falls on foul territory beyond first or third base, or
• first touches the person of an umpire, player, or any object foreign to the natural ground while on or over foul ground, or
• passes out of the playing field in flight while over foul ground.

When any foul or fair batted ball is caught in flight, the batter is out. When a batter bunts foul with two strikes, he is out. Otherwise, when a batted ball becomes a foul ball, the ball is dead, all runners must return to their time-of-pitch base, and the batter continues to bat.

It is common for a ball moving in or over fair territory to become a foul ball. Batted balls can also be fair balls or foul tips.

In different situations, a foul ball may be considered a positive or negative outcome of a pitch or swing. When there are zero or one strikes, a foul ball counts as a strike, benefitting the pitcher. However, a foul ball may reveal to the batter that he has timed a pitch well and need only make adjustment to the location of his swing on the next such pitch; this is often called a good cut or simply a good swing. Foul balls with two strikes are generally considered positive for the batter, since he thus avoids strike three on a potentially difficult pitch. Also, foul balls with two strikes increase the pitcher's pitch count, adding to his/her fatigue, thus providing some small advantage to the offense.