A term used to label a game that is released for more than one platform. Completely opposite from exclusive games (mostly first party titles). Movie games are often multi-platform because, in theory, everyone watches movies.
But, a game doesn't necessarily have to be on half a dozen different platforms to be "multi-platform." Most today are only on four or less. When Capcom makes a game for just the Gamecube and PS2, it's still multi-platform.