In baseball, a walk-off home run is a home run which ends the game. It must be a home run that gives the home team the lead in the bottom of the final inning of the game — either the ninth inning, any extra inning, or any other regularly scheduled final inning. It is called a "walk-off" home run because the teams walk off the field immediately afterward. Sportscasters also use the term "walk-off double" or other such terms if such a hit drives in the winning run to end the game. The terms walk-off hit by pitch or walk-off balk have been applied, and the latter has been dubbed a balk-off (these types of questionable walk-offs are seen by some fans as cheapening the concept). Although the concept is as old as baseball, the term itself has come into wide use only since the 1990s.
Outside of baseball, the term "walk-off" has been used recently in both college and pro football, using such terms as "walk-off field goal" and "walk-off touchdown", meaning a team scored the winning points to end the game either in sudden-death overtime or in the bottom half of the overtime period in college football.

History and usage of the term
In Japan, a walk-off home run is known as a sayonara home run. [3]

Japan
A technicality of the walk-off home run is that the game is not officially over until the winning run crosses home plate (in the case of a solo walk-off home run, the batter must round all the bases). This fact almost caused a serious problem in the 1976 ALCS. Mark Littell of the Kansas City Royals served up a home run ball to Chris Chambliss of the New York Yankees, who hit the home run that won the pennant. When jubilant Yankee fans ran onto the field at Yankee Stadium (the Yankees had not won the pennant in 12 years), preventing Chambliss from rounding the bases, Chambliss had to negotiate a sea of fans in order to place his foot in the area of home plate. Announcer Bill White, on WMCA radio in New York, yelled into the microphone, in a voice of disbelief, "...and the game...I THINK...is over!"
Another example is Robin Ventura's "Grand Slam Single" in the 1999 National League Championship Series. In the bottom of the 15th inning, the New York Mets tied the score against the Atlanta Braves at 3-3. Ventura came to bat with the bases loaded, and hit a walk-off grand slam to deep right. Roger Cedeño scored from third and John Olerud appeared to score from second, but Todd Pratt, on first base when Ventura hit the home run, went to second, then turned around and hugged Ventura, as the rest of the team piled onto the field. The official ruling was that because Ventura never advanced past first base, it was not a home run but a single, and thus only Cedeño's run counted, making the official final score 4-3.

Postseason and All-Star Game
In the charts below, home runs that ended a postseason series are denoted by the player's name in bold.
Follow the linked year on the far left for detailed information on that series.

World Series

Other postseason series

Regular season (selected examples)