What is the meaning of Top_secret?

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.

A level of security clearance needed to access such information.

Pertaining to any especially sensitive closed-room activities.

A secret that is classed as "top secret"; top-secret information.

Used other than figuratively or idiomatically: see top,‎ secret.

Source: wiktionary.org

See also