The term "walk-off" can only be used when the home team actually comes from behind in the 9th inning or later. By definition, any time a home team scores the winning run in its final at bat, its a walk-off, so let's reserve the term for moments that actually created real drama and excitement.
Under this new rule, the Marlins announcers would have had to refrain from using the term last night because Florida allowed the Orioles to tie the game in the 9th and the score remained tied until the Marlins scored the winning run in the bottom of the 12th. At no point did the Marlins ever trail Baltimore after the 8th inning. Ergo, walk-off does not apply. If the Orioles had taken the lead at some point in the 9th or later, then the Marlins announcers would have been free to use the term.
< /curmudgeon mode>