Entity

Either a subject (an active element that operates on information or the system state) or an object (a passive element that contains or receives information).
SOURCE: SP 800-27

An active element in an open system.
SOURCE: FIPS 188

Any participant in an authentication exchange; such a participant may be human or nonhuman, and may take the role of a claimant and/or verifier.
SOURCE: FIPS 196