In baseball, a complete game (CG) is the act of a pitcher pitching an entire game without the benefit of a relief pitcher. A pitcher who meets this criterion will be credited with a complete game regardless of the number of innings played—pitchers who throw an entire official game that is shortened by rain will still be credited with a complete game, while starting pitchers who are relieved in extra innings after throwing nine or more innings will not be credited with a complete game. A starting pitcher who is replaced by a pinch hitter in the final half inning of a game will still be credited with a complete game.

In baseball and softball, a relief pitcher or reliever is a pitcher who pitches in the game after the starting pitcher has been removed because of fatigue, ineffectiveness, injury, or ejection, or for other strategic reasons, such as inclement weather delays or pinch hitter substitutions. Relief pitchers are further divided informally into various roles, such as closers, setup men, middle relief pitchers, left/right-handed specialists, and long relievers. Whereas starting pitchers usually throw so many pitches in a single game that they must rest several days before pitching in another, relief pitchers are expected to be more flexible and typically pitch in more games with a shorter time period between pitching appearances but with fewer innings pitched per appearance. A team's staff of relievers is normally referred to metonymically as a team's bullpen, which refers to the area where the relievers sit during games, and where they warm-up prior to entering the game.