A Primitive element identifies a predefined data type, without any relevant substructure (that is, it has no parts in the context of UML). It could be regarded as a conceptual Data Type. The Primitive element can be used to support the Meta-Object Facility (MOF) specification.
OMG UML Specification:
The OMG Unified Modeling Language specification, (v2.5.1, p.168) states:
A PrimitiveType defines a predefined DataType, without any substructure. A PrimitiveType may have algebra and operations defined outside of UML, for example, mathematically. The run-time instances of a PrimitiveType are values that correspond to mathematical elements defined outside of UML (for example, the Integers).