The term polymorphism comes from greek roots. It means, roughtly, many forms. Readers of Classical literature might remember the story in Ovid of Morphus, who could appear to a sleeping person in a variety of different froms. In biology, a polyumorphic species is one that can appear in many different forms, such as human beings. In chemistry the term is used to refer to a substance that can crystalize in at least two distinct forms, such as carbon that can appear as graphite or as a diamond or, more exotically, as buckminister fullerenes.
It is perhaps appropriate that in computer science the term is used for a variety of different mechanisms.