The Collaborative International Dictionary of English v.0.48:

Classify \Clas"si*fy\, v. t. [imp. & p. p. Classified; p. pr.
   & vb. n. Classifying.] [L. classis class + -fy.]
   To distribute into classes; to arrange according to a system;
   to arrange in sets according to some method founded on common
   properties or characters.

   Syn: To arrange; distribute; rank.
        [1913 Webster]

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):

classified
    adj 1: arranged into classes [ant: unclassified]
    2: official classification of information or documents; withheld
       from general circulation; "thousands of classified documents
       have now been declassified" [ant: unclassified]
    n 1: a short ad in a newspaper or magazine (usually in small
         print) and appearing along with other ads of the same type
         [syn: classified ad, classified advertisement,
         classified]

Moby Thesaurus II by Grady Ward, 1.0:

76 Moby Thesaurus words for "classified":
   aligned, arcane, arranged, arrayed, assorted, cabalistic,
   cataloged, categorized, censored, close, closed, composed,
   concealed, constituted, cryptic, dark, disposed, enigmatic,
   esoteric, filed, fixed, graded, grouped, harmonized, hermetic,
   hidden, hierarchic, hush-hush, indexed, latent, marshaled,
   methodized, mysterious, normalized, occult, on file, ordered,
   orderly, organized, pigeonholed, placed, pyramidal, ranged, ranked,
   rated, regularized, regulated, restricted, routinized, secret,
   smothered, sorted, standardized, stifled, stratified, suppressed,
   synchronized, systematized, tabular, top secret, ulterior,
   unbreatheable, under security, under wraps, undisclosable,
   undisclosed, undivulgable, undivulged, unrevealable, unrevealed,
   unspoken, untellable, untold, unutterable, unuttered,
   unwhisperable