Entity
NIST SP 800-27rA
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).
NIST SP 800-33
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).
NIST SP 800-57P1
An individual (person), organization, device or process.
NIST SP 800-89
An individual (person), organization, device or process. Used interchangeably with "party".