Reference Monitor definition in Computer Security terms:
Definition(s): A set of design requirements on a reference validation mechanism which as key component of an operating system, enforces an access control policy over all subjects and objects. A reference validation mechanism must be: (i) always invoked (i.e., complete mediation); (ii) tamperproof; and (iii) small enough to be subject to analysis and tests, the completeness of which can be assured (i.e., verifiable).
Source(s): NIST SP 800-53 Rev. 4
The security engineering term for IT functionality that (1) controls all access, (2) cannot be by-passed, (3) is tamper-resistant, and (4) provides confidence that the other three items are true.
Source(s): NIST SP 800-33
reference: CSRC Glossary