A substitute is a player who comes into the game to replace another player.
A substitute who replaces a batter is known as a pinch hitter. If he replaces a baserunner, he is called a pinch runner. If he replaces the pitcher, he is a relief pitcher. If he replaces a fielder, he is called a defensive substitute.
Every time a substitute is used, the player he replaces must come out of the game and can no longer be used. There used to be exceptions to this rule, such as the use of courtesy runners, but these no longer exist except in exhibition games. There have been extremely rare cases when umpires have allowed a player to re-enter the game after having been replaced as a result of losing track of the various changes of players made by a team, but this is clearly a mistake not provided for by the rules.
A substitute must be on the team's roster before being allowed to enter the game. Every time a substitute is used, the home plate umpire must be advised and must allow the substitution. In certain cases, the umpire may have to check the eligibility of a player to determine whether he has been properly added to the roster or is serving a suspension. A substitution can only be made while the play is stopped.
A substitute may enter a game and be in turn substituted for before play has resumed. For example, if the opposing manager responds to a pinch hitter by bringing in a new pitcher, the pinch hitter may be replaced by a second pinch hitter who is more appropriate to the new situation. A substitute may also be replaced if he injures himself during warm-up. In any such case, the substitute is credited with a game played even if he did not participate in any play, and may not reenter the game at a later point.
Larry Yount, the brother of Hall of Famer Robin Yount made the only Major League appearance of his career with the 1971 Houston Astros when he warmed up after being brought into a game as a relief pitcher. He aggravated a pre-existing injury during his warm-up and was replaced before throwing a single pitch.