Defensive substitution

A defensive substitution in the game of baseball occurs when a currently non-playing player is placed into the field in place of another player, typically due either to injury or the appearance of a pinch hitter.
In the American League, a pinch hitter is usually substituted for the position player in whose place he batted. In the National League, pinch hitters sometimes bat for pitchers. However, another common pitcher-substitution strategy in the NL is the double switch, in which a pitching change is coupled with a pinch-hitter who is also defensively substituted for a second player.