This is a baseball phrase defined as a game in which the pitcher has pitched a minimum of nine innings without allowing a single runner on base. This means the pitcher may not allow any hits or any walks. By definition, a pitcher who accomplishes this feat has also pitched a no-hitter (also known as a "no-no") and a shutout. The pitcher requires the help of the defense, who must get out any batter who hits the ball by either catching it on the fly or getting them out at first base. If an error by a defensive player allows a player to reach base, the pitcher does not record a perfect game. However, if the error occurs in foul territory, it does not count against the pitcher.
A perfect game is an extremely difficult feat to achieve; it has only occurred eighteen times since 1900 and twenty in the history of Major League Baseball. Roy Halladay, Philip Humber, Matt Cain, and Felix Hernandez have recently joined the likes of Sandy Koufax to pitch a perfect game. Although a perfect game can be pitched by a combination of pitchers, all perfect games recorded in Major League history have been by a single pitcher. Many baseball fans are superstitious about using the phrase "perfect game" while it is occurring, as they believe it "jinxes" the game and hurts the pitcher's potential of completing the feat.
Also see: The "Perfect" Perfect Game