An individual or a group posing a threat.
Sources:
NIST SP 800-150
under Threat Actor
See threat actor.
Sources:
NIST SP 800-150
under Actor
The source of risk that can result in harmful impact.
Sources:
NIST SP 800-221
The instigators of risks with the capability to do harm.
Sources:
NISTIR 8286
under Threat Actor