Information System User (ISU) definition in Computer Security terms:
Definition(s): Individual, or (system) process acting on behalf of an individual, authorized to access an information system.
[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.].
Source(s): NIST SP 800-128 (CNSSI 4009)
reference: CSRC Glossary