Huh, I always assumed a walk-off home run was one hit on the first pitch to a batter, but Wikipedia encyclopedia has it:
"In baseball, a walk-off home run is a home run 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, or any other regularly scheduled final inning. It is called a "walk-off" home run because the teams walk off the field immediately afterward."

RE