A 1-2-3 inning refers to an inning in which a pitcher retires the batters in consecutive order. In a 1-2-3 inning, no batters reach base. This means that if the first batter reaches base, and the second batter grounds into a double play, the inning does not qualify as a 1-2-3 inning, even if the third batter gets put out. Each batter must be put out, either by strikeout or defensive put-out, without safely reaching base before the next batter comes to the plate.
