Security

Security is being free from danger. In absolute sense this is hardly possible, it is a relative matter. The term can be used with reference to crime, accidents of all kinds, etc.

Note that Security should not be confused with Safety. In some languages (e.g. German, Swedish ...) both meanings are represented by the same word. In Hebrew language security is a name of action ( אבטחה , avtacha ) which sould lead to safety ( ביטחון , bitachon ); Both words are derived from the same root bet-tet-heth ב-ט-ח .

Finance

In finance, a security is an instrument traded in the financial markets. See Securities.

Private security

In private security, security is the result of services provided to prevent, deter, detect and/or document crime, fire, disorder or violations of company rules. These services may be provided by a security guard or by alarms, security cameras, or as a routine part of the performance of every employee's duties (especially in smaller companies).

Compare with: National security, Police

Telecommunications

In telecommunication, 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.

Sources: from Federal Standard 1037C and adapted from the Department of Defense Dictionary of Military and Associated Terms and from the Department of Defense Dictionary of Military and Associated Terms

Another proposed alternative definition:

When our expectations are met, we can say that quality has been met. When our expectations are met once and again, despite of errors, catastrophes and attacks which in principle could prevent our expectations to be met, we can say that security has been met. Security is not falsable (Popper). We can prove that there has been a security failure, but we can´t prove that there hasn't. Security measures improve the likeliness of expectations to be met, and therefore improve security. With respect to classified matter there is an expectation of the classified matter to remain secret for as long as we wish. A control access system is the security measure that helps this expectation to be accomplished.