1. In computer and communications security , the central part of a computer or communications system hardware, firmware, and software that implements the basic security procedures for controlling access to system resources. 2. A self-contained usually small collection of key security-related statements that (a) works as apart of an operating system to prevent unauthorized access to, or use of, the system and (b) contains criteria that must be met before specified programs can be accessed. 3. Hardware, firmware, and software elements of a trusted computing base implementing the reference monitor concept. Security kernel must mediate all accesses, be protected from modification, and be verifiable as correct. [INFOSEC-99]4.The hardware, firmware, and software elements of a Trusted Computing Base that implement the reference monitor concept. It must mediate all accesses, be protected from modification, and be verifiable as correct [TCSEC].