The term 'walk off' refers to when the home team wins the game in the ninth inning or a later inning. It doesn't have anything to do with the type of play but simply that whatever play occurred was the play that scored the winning run for the home team. You can have a walk off base bit (single, double, triple, or home run), a walk off walk, a walk off hit by pitch, a walk off error, even a walk off balk.