Classified information is secret information to which access is restricted by law or corporate rules to a particular hierarchical class of people. A security clearance is permission to view a particular classified document or class of information, often requiring a satisfactory background check. This sort of hierarchical system of secrecy is used by virtually every national government, and by many corporations as well.
The desired degree of secrecy about such information is known as its sensitivity. It is often the case that sensitive information is disseminated on the basis of need-to-know, the assertion that if an individual needs to know certain information in order to satisfactorily perform her or his function in the organization, their viewing of that information is authorized, otherwise it is not.
Such information is called "classified" because it falls into a certain classification of secrecy. Information which isn't secret is called unclassified information, which carries the doublespeak implication that the natural state of information is to be classified, in other words, to be made secret.
The United States government, for example, has a formal hierarchy of classification for secrets:
- Top secret – this is the highest security level, and is defined as information which would cause "exceptionally grave damage" to national security if disclosed to the public. This classification is most often subdivided on the basis of "need to know", and includes such information as the design of cutting-edge weaponry, etc.
- Secret – the second highest classification may include, for example, details of other security measures and procedures. It is defined as information which would cause "serious damage" to national security if disclosed.
- Confidential – is the lowest classification level. It is defined as information which would "damage" national security if disclosed.
- Sensitive but Unclassified (SBU) – data which is not related to national security but whose disclosure to the public could cause some harm; such data includes personal demographic information from recent censuses, for example.
- Unclassified – not technically a "classification", this is the default, and refers to information that is not sensitive and can be freely disclosed to the public. Information which was previously classified under one of the above levels is often declared "unclassified" at a certain time because its age has made its classification no longer necessary.
The classification scheme of course varies between organizations; for example, in Canada information which the U.S. would classify SBU is called "protected," and further subcategorized into levels A, B, and C.