If a software program is developed for mulitple operating systems, it is considered to be "multiplatform." Since Microsoft Word runs on both the Windows and Macintosh platform, it is a mutliplatform application.
In the consumer gaming market, mutliplatform games run on more than one gaming machine. For example, a sports game developed for Xbox, Playstation, GameCube, and PC would be a multiplatform game. If a game is developed exclusively for one system, i.e. "The Legend of Zelda," for Nintendo, it is not multiplatform. Gaming hardware manufacturers use exclusive software as a reason for consumers to buy their system.