<programming> A property of a data-object that can exist in many shapes and sizes, but not simultaneously; which distinguishes it from a union. It is often implemented as a set of classes (or structs) derived from a common base class (or with a common header, as in the case of structs), typically without any methods.
It has been loosely described as polymorphic data.
Try this search on Wikipedia, OneLook, Google
Nearby terms: Poly « polygon pusher « POLYGOTH « polylithism » Poly/ML » polymorphic » polymorphic lambda-calculus