The Collaborative International Dictionary of English v.0.48:

classified \classified\ adj.
   1. arranged into classes or categories; as, unclassified.

   Syn: categorized.
        [WordNet 1.5 +PJC]

   2. assigned to a class of documents withheld from general
      circulation; -- of information or documents. Opposite of
      unclassified.

   Note: [Narrower terms: eyes-only; confidential;
         restricted; secret; sensitive; top-secret]
         [WordNet 1.5]

WordNet (r) 3.0 (2006):

top-secret
    adj 1: the highest official level of classification of documents