A perfect game is defined by Major League Baseball as a game in which a pitcher (or combination of pitchers) pitches a victory that lasts a minimum of nine innings and in which no opposing player reaches base. Thus, the pitcher (or pitchers) cannot allow any hits, walks, hit batsmen, or any opposing player to reach base safely for any other reason—in short, "27 up, 27 down". The feat has been achieved only 18 times in the history of major league baseball—16 times since the modern era began in 1900.

By definition, a perfect game must be both a no-hitter and a shutout. Since the pitcher cannot control whether or not his teammates commit any errors, the pitcher must be backed up by solid fielding to pitch a perfect game. An error that does not allow a baserunner, such as a misplayed foul ball, does not spoil a perfect game. Weather-shortened contests in which a team has no baserunners and games in which a team reaches first base only in extra innings do not qualify as official perfect games under the present definition. The first confirmed use of the term "perfect game" was in 1908; the current official definition of the term was formalized in 1991. Although it is possible for multiple pitchers to combine for a perfect game (as has happened nine times at the major league level for a no-hitter), to date, every major league perfect game has been thrown by a single pitcher.