Security safeguards (i.e., hardware and software features, physical controls, operating procedures, management procedures, and various combinations of these) designed to detect and deny unauthorized access and permit authorized access to an information system.
Sources:
CNSSI 4009-2022

  As an implementation of formal access control policy based on a formal access control model, this is the logical component that serves to receive the access request from the subject, to decide, and to enforce the access decision. Access control mechanisms can be designed to adhere to the properties of the model by machine implementation using protocols, architecture, or formal languages such as program code.
Sources:
CNSSI 4009-2022 from NIST SP 800-162 - adapted, NIST SP 800-192 - adapted

  Implementations of formal AC policy such as AC model. Access control mechanisms can be designed to adhere to the properties of the model by machine implementation using protocols, architecture, or formal languages such as program code.
Sources:
NIST SP 800-192 under Access Control Mechanism

  The logical component that serves to receive the access request from the subject, to decide, and to enforce the access decision.
Sources:
NIST SP 800-162