An Enumeration is a data type, whose instances can be any of a number of user-defined enumeration literals. It is possible to extend the set of applicable enumeration literals in other packages or profiles. You create Enumerations in Class or Package diagrams, and in diagrams developed using the Metamodel and Profile pages of the Diagram Toolbox.
OMG UML Specification:
The OMG UML specification (UML Superstructure Specification, v2.1.1, p. 69) states:
An enumeration is a data type whose values are enumerated in the model as enumeration literals.