A Device is a physical electronic resource with processing capability upon which Artifacts can be deployed for execution, as represented in a Deployment diagram. Complex Devices can consist of other devices; that is, a Device can be a nested element, where a physical machine is decomposed into its elements either through namespace ownership or through attributes that are typed by Devices.
OMG UML Specification:
The OMG Unified Modeling Language specification, v2.5.1, p.663) states:
A device is a physical computational resource with processing capability upon which artifacts may be deployed for execution. Devices may be complex (i.e., they may consist of other devices).