The Carribean Plate (I assume that's the one you mean) is largely an oceanic plate, making it geologically distinct from the more continental North and South Amercian plates. The other continents have all pretty much moved independently of each other, including the Americas, while land on the Carribean Plate is largely the result of it being 'pinched' between North and South America. Plates are more of a cause of continents than a defining factor, but they can serve to be a good indicator. In this case though, it means that sometime in the geologically near future, North and South America will no longer be connected at all, and likely most of Central Amercia will be underwater. I think most people would define continents to avoid having them disappear (though Antartica will be a different issue should all the ice melt).