In the Unified Modeling Language 1.x, powertype is a keyword for a specific UML stereotype, and applies to a class or dependency. Powertype shows a classifier whose instances (objects) are children of the given parent.

In UML 2.x, a powertype is a metaclass whose instances are subclasses of a given class. The stereotype has been removed and the powertype is now indicated by typing the generalization set.

In the UML Superstructure 2.4.1 Specification Document^[1] the following definition is given:

The notion of power type was inspired by the notion of power set. A power set is defined as a set whose instances are subsets. In essence, then, a power type is a class whose instances are subclasses.

In UML 2.5.1,^[2] the formal definition of powertypes in terms of power sets (or sets of subsets) are no longer given. Instead, powertypes are illustrated through various examples.