An immaculate inning in baseball is a rare and impressive feat where a pitcher strikes out all three batters he faces in a single inning using the minimum possible number of pitches: nine. This means every pitch thrown is a strike, with no balls or fouls extending the at-bat, and each batter is retired on exactly three pitches each. The pitcher throws exactly nine pitches, all strikes, resulting in three strikeouts and three outs for the inning. The key elements are:

• Three batters faced
• Each batter struck out on three pitches
• A total of nine pitches thrown
• All pitches result in strikes (no balls or foul balls prolonging the at-bat)
• Three outs recorded

This demonstrates skill, precision, and dominance by the pitcher, as it leaves no room for error or luck. It is often celebrated as a remarkable display of pitching mastery. The first recorded immaculate inning was thrown on June 4, 1889, by John Clarkson. Since then, it has been achieved 116 times in Major League Baseball history as of 2024. Though still rare, it has become more frequent in recent decades due to changes in the game. An immaculate inning can occur even with runners on base, provided the pitcher records three strikeouts on nine pitches. However, if a batter reaches base due to a dropped third strike, it is not considered immaculate. In summary, an immaculate inning is striking out the side on nine pitches — three batters, three strikeouts, nine pitches total — a perfect and efficient inning for a pitcher. It is highly regarded as a demonstration of pitching excellence.