top secret

Meanings

1. 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.
2. A level of security clearance needed to access such information.
3. Pertaining to any especially sensitive closed-room activities.

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