top secret

Short definition from Wiktionary

Adjective

1. 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. 2
   A level of security clearance needed to access such information.
3. 3
   Pertaining to any especially sensitive closed-room activities.
4. 4
   Having one of the highest secrecy classifications, "top secret".

Noun

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