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Individual, or (system) process acting on behalf of an individual, authorized to access a system.
NIST SP 800-171 Rev. 2
NIST SP 800-37 Rev. 2
An individual or (system) process acting on behalf of an individual that is authorized to access information and information systems to perform assigned duties. Note: With respect to SecCM, an information system user is an individual who uses the information system functions, initiates change requests, and assists with functional testing.
NIST SP 800-128 from NIST SP 800-37