A baseball player on the team at bat who is on base (or attempting to reach a base) WordNet 3.0 & Open English WordNet

A player who is on base and is thus trying to advance to the next base. Wiktionary

Player. Wiktionary (translation)

A person (from one or the other team) who runs out onto the field during the game to take verbal instructions from the coach to the players. A runner mustn't interfere with play, and may have to wear an identifying shirt to make clear his or her purpose on the field. Wiktionary