top secret

CIDE DICTIONARY

top secret, a. 

•  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.

  "The names of the higher levels of secrecy classification are themselves classified secrets."  [PJC]
•  kept secret by very careful and strict rules for disclosure.  [PJC]

THESAURUS

top secret

arcane, cabalistic, censored, classified, close, closed, concealed, cryptic, dark, enigmatic, esoteric, hermetic, hidden, hush-hush, latent, mysterious, occult, restricted, secret, smothered, stifled, suppressed, ulterior, unbreatheable, under security, under wraps, undisclosable, undisclosed, undivulgable, undivulged, unrevealable, unrevealed, unspoken, untellable, untold, unutterable, unuttered, unwhisperable

ROGET THESAURUS

top secret

Secret

N secret, dead secret, profound secret, arcanum, mystery, latency, Asian mystery, sealed book, secrets of the prison house, le desous des cartes, enigma, riddle, puzzle, nut to crack, conundrum, charade, rebus, logogriph, monogram, anagram, Sphinx, crux criticorum, maze, labyrinth, Hyrcynian wood, intricacy, meander, problem, paradox, unintelligibility, terra incognita, secret, involved, labyrinthine, labyrinthian, mazy, confidential, top secret.