X-ray machines and metal detectors are used to control what is allowed to pass through an airport security perimeter.
Security spikes protect a gated community in the East End of London.Security is the degree of protection against danger, loss, and criminals. Individuals or actions that encroach upon the condition of protection are responsible for a "breach of security."

A condition that results from the establishment and maintenance of protective measures that ensure a state of inviolability from hostile acts or influences.
Security has to be compared and contrasted with other related concepts: Safety, continuity, reliability. The key difference between security and reliability is that security must take into account the actions of people attempting to cause destruction.