AbstractThe specification and design of modern security systems are hampered by terminology that is overlapping and recursive. The definitions offered by prominent standards bodies lack commonality in meaning and interpretation and tend to be specific to electronic or cyber security. Consequently, the current set of security terms and definitions is of little use to stakeholders. This paper begins by examining the definitions and terms applied to security and security systems. A systems engineering approach is then used to analyse the set of terms and to propose a new definition of security, from which a suitable set of security terms is decomposed. Definitions of security services and security mechanisms that have a broad application across the electronic, physical, and personnel security domains are then developed, and examples are provided that illustrate the utility of the set of definitions in security management, requirements engineering, systems engineering, and system design methodologies.