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An operating system in which there exists a level of confidence (based on rigorous analysis and testing) that the security principals and mechanisms (e.g., separation, isolation, least privilege, discretionary and non-discretionary access control, trusted path, authentication, and security policy enforcement) are correctly implemented and operate as intended even in the presence of adversarial activity.
CNSSI 4009-2015 CNSSI 1253
An operating system that manages data to make sure that it cannot be altered, moved, or viewed except by entities having appropriate and authorized access rights.
NIST SP 800-152 under Trusted (secure) operating system