1.AdjectiveInformation 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.
"NSA's report on the Liberty incident was originally classified top secret."
2.AdjectiveA level of security clearance needed to access such information.
3.AdjectivePertaining to any especially sensitive closed-room activities.
"Steve Jobs set up a top secret team to design the Macintosh."
4.NounA secret that is classed as "top secret"; top-secret information.
5.NounUsed other than figuratively or idiomatically: see top, secret.