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.
OMG UML Specification
The OMG UML specification (UML Superstructure Specification, v2.1.1, p. 124) states:
A primitive data type may have an algebra and operations defined outside of UML, for example, mathematically ... The run-time instances of a primitive type are data values. The values are in many-to-one correspondence to mathematical elements defined outside of UML (for example, the various integers). Instances of primitive types do not have identity. If two instances have the same representation, then they are indistinguishable.