The term 'Caller Strike' refers to a situation in which an UMPIRE, or UMP (3 letters), calls a strike on a pitch. An UMPIRE is an official who makes decisions about the rules of a game, such as whether a pitch is a ball or a strike. In baseball, the UMPIRE is usually positioned behind the catcher and is responsible for making the call on each pitch. A referee, or REF (3 letters), is another type of official who is responsible for enforcing the rules of a game. In baseball, the referee is usually positioned near the pitcher's mound and is responsible for ensuring that the players follow the rules.