A "walk-off" is any offensive play that gives the home team the lead -- and thus, the win -- in the bottom of the last inning.
The term walk-off originated as "walk-off piece," and was coined by Hall of Famer Dennis Eckersley. ''It was always walk-off piece," Eckersley told the Boston Globe. "Like something you would hang in an art gallery. The walk-off piece is a horrible piece of art."
The first reference to walk-off came in a July 30, 1988, story in the Gannett News Service: "In Dennis Eckersley's colorful vocabulary, a walk-off piece is a home run that wins the game and the pitcher walks off the mound."
Walk-off piece was only intended to describe a pitcher's dejected walk off the field after giving up a game-losing home run, but it soon grew into its own phenomenon.