A substitution was the choosing of an alternate player in place of an athlete who had been removed from the game. In baseball, a player who hit the ball in place of another was said to be pinch-hitting.
In 2375, upon hearing that Rom wanted to play in the game, Benjamin Sisko told Miles O'Brien to call a timeout to make a substitution, letting Rom hit next. (DS9: "Take Me Out to the Holosuite")