In baseball, the statistic innings pitched (IP) is the number of innings a pitcher has completed, measured by the number of batters and baserunners that have been put out while the pitcher is on the pitching mound in a game. Three outs made is equal to one inning pitched. One out counts as one-third of an inning, and two outs as two-thirds of an inning. Sometimes, the statistic is written 34.1, 72.2, or 91.0, for example, to represent 34+1⁄3 innings, 72+2⁄3 innings, and 91 innings exactly, respectively.

In baseball, softball, and similar games, an inning is the basic unit of play, consisting of two halves or frames, the "top" and the "bottom". In each half, one team bats until three outs are made, with the other team playing defense. A full baseball game is typically scheduled for nine innings, while softball games consist of seven innings, although this may be shortened due to weather or extended if the score is tied at the end of the scheduled innings. The use of the term inning in baseball and softball contrasts with cricket and rounders, in which the term is innings in both singular and plural.