In baseball

Baseball

Baseball is a bat-and-ball sport played between two teams of nine players each. The aim is to score runs by hitting a thrown ball with a bat and touching a series of four bases arranged at the corners of a ninety-foot diamond...

, a walk-off home run is a home run

Home run

In baseball, a home run is scored when the ball is hit in such a way that the batter is able to reach home safely in one play without any errors being committed by the defensive team in the process...

 that ends the game. It must be a home run that gives the home team the lead (and consequently, the win) in the bottom of the final inning of the game—either the ninth inning, or any extra inning

Extra innings

Extra innings is the extension of a baseball or softball game in order to break a tie.Ordinarily, a baseball game consists of nine innings , each of which is divided into halves: the visiting team bats first, after which the home team takes its turn at bat...

, or any other regularly scheduled final inning. It is called a "walk-off" home run because both teams walk off the field immediately afterward, rather than finishing the inning.
Although the concept of a game-ending home run is as old as baseball, the adjective "walk-off" only attained widespread use in the late 1990s and early 2000s.