[andreyf]

Actually, polymorphism is when two or more clearly different phenotypes exist in the same population of a species — in other words, the occurrence of more than one form or morph. </tounge-in-cheek>

Polymorphism is certainly not a very well defined notion. Personally, I think the best answer would be "I don't know a precise definition, I've heard it mean different things based on context, for example...".