Not open to the public; not available to or shared by the public; private.
A level of security clearance needed to access such information.
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.
Pertaining to any especially sensitive closed-room activities.
A secret that is classed as "top secret"; top-secret information.
Used other than figuratively or idiomatically: see top, secret.