An active entity, generally in the form of a person, process, or device, that causes information to flow among objects or changes the system state.
NIST SP 800-33
NIST SP 800-27 Rev. A [Withdrawn]
Generally an individual, process, or device causing information to flow among objects or change to the system state. See object.
(NIST SP 800-53 Rev. 4)
NIST SP 800-53 Rev. 4 under Subject
the set of active entities of the system, operating within roles on behalf of individual users.
NISTIR 6192 under Subject
A person, organization, device, hardware, network, software, or service.
NIST SP 800-63-3 under Subject