Adjective

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

   NSA's report on the Liberty incident was originally classified top secret.

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

   Steve Jobs set up a top secret team to design the Macintosh.