In baseball, a walk-off home run is a home run that ends the game. For a home run to end the game, it must be hit in the bottom of the final inning and generate enough runs to exceed the opponent's score. Because the opponent will not have an opportunity to score any more runs, there is no need to finish the inning and the teams can walk off the field immediately. The winning runs must still round all three bases and be counted at home plate. A variant of the walk-off home run, the walk-off grand slam, occurs when a grand slam exceeds the opponent's score in the bottom of the final inning and ends the game.

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.