[Alex]

I pretty much agreed what others were saying, but I thought the walk-off referred to the losing pitcher, so I did some digging and this is what I found:

The term "walk off" should only be used when a game ends with the final batter hitting a game-ending (and game-winning) home run. Broadcasters now tend to use the term to describe any game-ending (and game-winning) hit be it a single, double, triple or home run.

Dennis Eckersley (the great relief pitcher) is credited with first using the term "walk off" as a way to describe a game that ends abruptly with the final batter hitting a clutch game-winning home run. He used the term to describe how the pitcher who gave up the game-ending (and game-winning) home run will now have nothing else to do than "walk off" the field in defeat.

So, this may or may not be accurate, but I am sticking to it.