A set of design requirements on a reference validation mechanism that, as a key component of an operating system, enforces an access control policy over all subjects and objects. A reference validation mechanism is always invoked (i.e., complete mediation), tamperproof, and small enough to be subject to analysis and tests, the completeness of which can be assured (i.e., verifiable).
Sources:
NIST SP 800-53 Rev. 5