- A foul ball is a batted ball that settles on foul territory between home and first base, or between home and third base, or that bounds past first or third base on or over foul territory, or that first falls on foul territory beyond first or third base, or that, while on or over foul territory, touches the person of an umpire or player, or any object foreign to the natural ground.
- A foul fly shall be judged according to the relative position of the ball and the foul line, including the foul pole, and not as to whether the infielder is on foul or fair territory at the time he touches the ball.
- Comment: A batted ball not touched by a fielder, which hits the pitcher's rubber and rebounds into foul territory, between home and first, or between home and third base is a foul ball.
A batted ball that is not a foul ball is a fair ball.
A foul ball may be either in play or out of play. It is in play if hit in the air in foul territory but can be caught by a player without leaving the field of play. For example, a fielder may reach with his glove into the third row of the stands to catch a foul ball, as long as his feet are still on the playing field, or he has jumped and not yet landed outside the field. If the fielder is no longer in play when making the catch, the ball is considered dead and the batter is not out.
A foul ball is considered a dead ball as soon as it hits the ground or leaves the field of play. In certain stadiums, ground rules state that hitting a speaker or another object located in foul territory will make a ball become dead on impact, regardless of whether it lands in the field of play afterwards.
No play can be made after a foul ball becomes a dead ball and baserunners must return to their original base. However, if a foul ball is caught before hitting the ground, runners can advance at their own risk as on any other fly ball that is caught.
The Fair-Foul Ball
In the 1870s, the rules governing foul balls were different. Any ball hit first into fair territory was considered fair, no matter where it rolled or bounced afterwards. A few players specialized in the art of "fair-foul hitting", which consisted in hitting or bunting the ball with such backspin that it would twist sharply into foul territory, far from where the defensive players were posted, thereby making it very difficult to field. The creation of this type of hitting was credited to Dickey Pearce, one of baseball's earliest stars, and was perfected by Ross Barnes, the best hitter in the National Association and in the first year of the National League. This style of hitting was controversial even at the time, and the rules were adjusted after the 1876 season to the current definition of a foul ball, making the fair-foul hit obsolete.
- Robert H. Schaefer: "The Lost Art of Fair-Foul Hitting", The National Pastime, SABR, Number 20 (2000), pp. 3-7.