It is one of the rarest feats in baseball, only happens on average once every five or six years, the perfectly thrown baseball game in the major leagues. It's a bit of a misnomer to begin with, as the perfect pitched game MUST include no errors by the defense behind the pitcher as well, over which he has little to no control, but the pitcher must not relinquish a single walk or hit to any of the 27-batters faced across the entire game.
As Matt Cain of the San Francisco Giants did this evening for just the 22nd time in Major League Baseball's well over one hundred year history of play, striking out an incredibly 14 of the 27 Houston Astros batters he faced, and with some help from at least one truly exceptional defensive play by Gregor Blanco in the outfield.
Perfection. It doesn't come around very often.