A person, organization, device, hardware, network, software, or service.
Sources:
NIST SP 800-63-3
under Subject
Generally an individual, process, or device causing information to flow among objects or change to the system state. See object.
Sources:
CNSSI 4009-2015
An individual, process, or device that causes information to flow among objects or change to the system state. Also see object.
Sources:
NIST SP 800-53 Rev. 5
NIST SP 800-53A Rev. 5
The entity requesting to perform an operation upon the object.
Sources:
NIST SP 800-162
the set of active entities of the system, operating within roles on behalf of individual users.
Sources:
NISTIR 6192
under Subject