Security service (telecommunication)

Security service is a service, supplied by a layer of communicating open systems, which ensures adequate security of the systems or of data transfers as identified by ITU-T X.800 Recommendation.
X.800 and ISO 7498-2 (Information processing systems – Open systems interconnection – Fundamental Reference Model – Part 2: Security design) are technically aligned. This model is broadly recognized

A capacity that supports one, or more, of the security requirements (Privacy, Integrity, Availability). Examples of security services are authentication, access control, and essential management.
Another definition that is important is in W3C Web service Glossary  adopted by NIST SP 800-95:

A security service is a superset of AAA services.