top secret

from the GNU version of the Collaborative International Dictionary of English.

• adjective (Mil.) containing information whose disclosure could result in grave danger to the national security; -- the highest of the three commonly known levels of national security classification, the others being confidential and secret.
• adjective kept secret by very careful and strict rules for disclosure.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective military 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.
• adjective A level of security clearance needed to access such information.
• adjective figuratively Pertaining to any especially sensitive closed-room activities.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• adjective the highest official level of classification of documents