top secret
adjective
Definitions
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.
Example: NSA's report on the Liberty incident was originally classified top secret.
A level of security clearance needed to access such information.
Pertaining to any especially sensitive closed-room activities.
Example: Steve Jobs set up a top secret team to design the Macintosh.