Security is the condition of being protected against danger or loss. In the general sense, security is a concept similar to safety. The nuance between the two is an added emphasis on being protected from dangers that originate from outside. Individuals or actions that encroach upon the condition of protection are responsible for the breach of security.

The word "security" in general usage is synonymous with "safety," but as a technical term "security" means that something not only is secure but that it has been secured. In telecommunications, the term security has the following meanings:

A condition that results from the establishment and maintenance of protective measures that ensure a state of inviolability from hostile acts or influences.
With respect to classified matter, the condition that prevents unauthorized persons from having access to official information that is safeguarded in the interests of national security.
Measures taken by a military unit, an activity or installation to protect itself against all acts designed to, or which may, impair its effectiveness.
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 active malicious agents attempting to cause destruction.

It is very often true that people's perception of security is not directly related to actual security. For example, a fear of flying is much more common than a fear of driving; however, driving is generally a much more dangerous form of transport. The tool may be mistaken for the effect, for example when multiple computer security programs interfere with each other, so the user assumes the computer is secure when actual security has vanished.

Another side of this is a phenomenon called security theatre where ineffective security measures such as screening of airline passengers based on static databases are introduced with little real increase in security or even, according to the critics of one such measure - Computer Assisted Passenger Prescreening System - with an actual decrease in real security.

Additionally, however, sometimes if it is perceived that there is security then there will be an increase in actual security, even if the perception of security is mistaken. Sometimes a sign may warn that video surveillance is covering an area, and even if there is no actual visual surveillance then some malicious agents will be deterred by the belief that there may be. Also, often when there is actual security present in an area, such as video surveillance, an alarm system in a home, or an anti-theft system in a car such as a LoJack, signs advertising this security will increase its effectiveness, protecting the value of the secured vehicle or area itself. Since some intruders will decide not to attempt to break into such areas or vehicles, there can actually be less damage to windows in addition to protection of valuable objects inside. Without such advertisement, a car-thief might, for example, approach a car, break the window, and then flee in response to an alarm being triggered. Either way, perhaps the car itself and the objects inside aren't stolen, but with perceived security even the windows of the car have a lower chance of being damaged, increasing the financial security of its owner(s). It is important, however, for signs advertising security not to give clues as to how to subvert that security, for example in the case where a home burglar might be more likely to break into a certain home if he or she is able to learn beforehand which company makes its security system.