In Major League Baseball (MLB), the 300 save club is the group of pitchers who have recorded 300 or more regular-season saves in their careers. Most commonly a relief pitcher earns a save by being the final pitcher of a game in which his team is winning by three or fewer runs and pitching at least one inning without losing the lead. The final pitcher of a game can earn a save by getting at least one batter out to end the game with the winning run on base, at bat, or on deck, or by pitching the last three innings without relinquishing the lead, regardless of score. The statistic was created by Jerome Holtzman in 1959 to "measure the effectiveness of relief pitchers" and was adopted as an official statistic by MLB in 1969. The save has been retroactively measured for past pitchers where applicable. Hoyt Wilhelm retired in 1972 and recorded just 31 saves from 1969 onwards, for example, but holds 228 total career saves.

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.