Secrecy is the practice of hiding information from certain individuals or groups who do not have the "need to know", perhaps while sharing it with other individuals. That which is kept hidden is known as the secret.
Secrecy is often controversial, depending on the content or nature of the secret, the group or people keeping the secret, and the motivation for secrecy.
Secrecy by government entities is often decried as excessive or in promotion of poor operation; excessive revelation of information on individuals can conflict with virtues of privacy and confidentiality. It is often contrasted with social transparency.
Secrecy can exist in a number of different ways: encoding or encryption (where mathematical and technical strategies are used to hide messages), true secrecy (where restrictions are put upon those who take part of the message, such as through government security classification) and obfuscation, where secrets are hidden in plain sight behind complex idiosyncratic language (jargon) or steganography.
Another classification proposed by Claude Shannon in 1948 reads that there are three systems of secrecy within communication: Concealment systems, including such methods as invisible ink, concealing a message in a harmless text in a fake covering cryptogram, or other methods in which the existence of the message is concealed from the enemy. Privacy systems, for example, voice inversion, is a special equipment required to recover the message. "True" secrecy systems where the meaning of the message is concealed by the cypher, code, etc. Although, its existence is not hidden. The enemy is assumed to have any special equipment necessary to intercept and record the transmitted signal.
Sociology
Animals conceal the location of their den or nest from predators. Squirrels bury nuts, hiding them, and they try to remember their locations later.
Humans attempt to consciously conceal aspects of themselves from others due to shame, or from fear of violence, rejection, harassment, loss of acceptance, or loss of employment. Humans may also attempt to conceal aspects of their own self which they are not capable of incorporating psychologically into their conscious being. Families sometimes maintain "family secrets", obliging family members never to discuss disagreeable issues concerning the family with outsiders or sometimes even within the family. Many "family secrets" are maintained by using a mutually agreed-upon construct (an official family story) when speaking with outside members. Agreement to maintain the secret is often coerced through "shaming" and reference to family honor. The information may even be something as trivial as a recipe.
Secrets are sometimes kept to provide the pleasure of surprise. This includes keeping secret about a surprise party, not telling spoilers of a story, and avoiding exposure of a magic trick.
Keeping one's secret strategy is important in many aspects of game theory.
In anthropology secret sharing is one way for people to establish traditional relations with other people. A commonly used narrative that describes this kind of behavior is Joseph Conrad's short story "The Secret Sharer".
Government
Governments often attempt to conceal information from other governments and the public. These state secrets can include weapon designs, military plans, diplomatic negotiation tactics, and secrets obtained illicitly from others ("intelligence"). Most nations have some form of Official Secrets Act (the Espionage Act in the U.S.) and classify material according to the level of protection needed (hence the term "classified information"). An individual needs a security clearance for access and other protection methods, such as keeping documents in a safe, are stipulated.
Few people dispute the desirability of keeping Critical Nuclear Weapon Design Information secret, but many believe government secrecy to be excessive and too often employed for political purposes. Many countries have laws that attempt to limit government secrecy, such as the U.S. Freedom of Information Act and sunshine laws. Government officials sometimes leak information they are supposed to keep secret. (For a recent (2005) example, see Plame affair.)