Non-official cover (NOC) is a term used in espionage (particularly by the CIA) for an agent or operative who assumes a covert role in an organization without ties to the government he or she is working for. It's typically abbreviated in espionage lingo as NOC (pronounced "knock").
An agent sent to spy on a foreign country might for instance pose as a journalist, a businessperson, a worker for a non-profit organization (such as a humanitarian group), or an academic. Nonofficial cover is contrasted with official cover, where an agent assumes a position at a seemingly benign department of their government, such as the diplomatic service. If caught, agents under nonofficial cover are usually trained to deny any connection with their government, and do not have many of the protections offered to (for example) accredited diplomats who are caught spying. Some countries have regulations regarding the use of nonofficial cover—the CIA, for example, has at times been prohibited from disguising agents as members of certain aid organizations, or as members of the clergy.
The degree of sophistication put into nonofficial cover stories can vary considerably. Sometimes, an agent will simply be appointed to a position in a well-established company which can provide the appropriate opportunities. Other times, entire front companies can be established in order to provide false identities for agents. Examples include Air America, used by the CIA during the Vietnam War, and made famous by the eponymous film. Another is Brewster Jennings & Associates, used by the CIA in WMD investigations, and made famous by the Plame Affair.