This is a baseball term which refers to a substitute who comes in for another player who is already on base. This substitution is usually made when the player on base is slow, has an injury that slows them down, or if the team needs to have a fast runner on base to attempt a steal. As with other substitution rules in baseball, the substitute must stay in the game and the player they replace may not come back in. The pinch runner will have to both hit and play the field, though they do not have to play the same position as the player they replace. The pinch runner may also be substituted out at any time, at the manager's discretion.
An interesting fact about a pinch runner is that they do not record a "game-played" in regards to consecutive games played. Also, prior to 1949, Major League Baseball employed the use of "courtesy runners" who could come on to run for a player if the player was temporarily injured or for other short term use. The substitute could be a player already in the starting line-up, or a new one, and the player replaced could come back in at anytime. One of the most famous pinch runners was track star Herb Washington, who was signed by the Oakland Athletics and never recorded a hit or played the field, but managed to steal 31 bases and score 33 times.