A term in baseball that refers to the area on the field, or diamond, left open as a result of defensive positioning. On a baseball diamond, there may be gaps created based on where the defensive players position themselves. Typically, a gap will exist between first and second base, and between shortstop and second base. In the outfield, gaps exist between center field and right or left field. Hitters are often trying to hit the ball into the gap since there are no defensive players there. To counter this, defensive players will often position themselves in the area of the field where the hitter is most likely to hit the ball. The pitcher will also try to pitch in a way that if there is a hit, the ball will go towards the defense.
There are many techniques for hitting the ball into the gap, pitching to prevent it, and fielding to close the gaps. Hitters will often try to "pull" the ball to the opposite field, where defense typically isn't expecting the ball. Pitchers will try to pitch the ball away from the batter's sweet spot, or inside/outside, depending on the strength of the hitter, to prevent the hitter from having control or power on the hit. Fielders will also move around during an at-bat to keep the batter guessing as to their final position.