Having one of the highest secrecy classifications, "top secret".

Information that is widely known, but not acknowledged openly. / (dated) Information that is not widely known, despite being freely available.

A secret that is classed as "top secret"; a top-secret secret; top-secret information. / Used other than figuratively or idiomatically: see top, secret.

(military, government) Information classified at one of the highest levels of sensitivity by a government, based on an assessment that it would cause exceptionally grave damage to national security if disclosed.

A member of an intelligence agency whose identity or affiliation is kept confidential.

A police force operating in secrecy and outside the normal boundaries of law and hidden from the public, usually in support of a totalitarian government's political policies to suppress political dissent through the use of methods including intimidation, violence, and surveillance.

One who admires another romantically but purposely remains anonymous to the extent that the subject of the admiration is unaware of the admirer's identity.

A Christmas tradition where a group of people give anonymous gifts to each other, with each person randomly selected to give a gift to one other person.

(business) A formula, practice, process, design, instrument, pattern, or compilation of information used by a business to obtain an advantage over competitors within the same industry or profession, which kept secret from those competitors or the public.

In Australian Aboriginal culture, restricted to initiated men, or to women; not public knowledge.