A perfect game in baseball is when no one on the opposing team ever reaches first base. It requires that every one of the 27 batters be retired with an out. Unlike a "no-hitter", a perfect game cannot have any walks or errors.
As of 2012, there have been only 20 perfect games in major league baseball history, out of hundreds of thousands of games. At least two of those games (including the only World Series perfect game) have ended on questionable calls by the home plate umpire.
Phil Humber pitched the 20th perfect game in major league baseball history in April 2012, but he threw 7 more pitches than Great Conservative Sports Star Jim Bunning's 90-pitch perfect game four decades ago.