Immaculate innings are those innings in baseball games where the pitcher strikes out all three batters with nine pitches. It's only happened 42 times in the history of the game. David Archer is tracking the frequency of such innings; they are getting more common.
As one friend pointed out, the best explanation for the increase in recent decades appears to be the advent of the modern reliever, especially the flame-throwing, one inning closer (more immaculate innings have been thrown in the 9th inning* (eight) than in any other inning), though starters -- such as Burnett -- have also been throwing them with impressive frequency.
Update: See also three-pitch innings. (via noah brier)