The Collaborative International Dictionary of English v.0.48:

Top secret \Top" sec`ret\, a.
   1. (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.

   Note: The names of the higher levels of secrecy
         classification are themselves classified secrets.
         [PJC]

   2. kept secret by very careful and strict rules for
      disclosure.
      [PJC]

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "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