The Collaborative International Dictionary of English v.0.48:

Confidential \Con`fi*den"tial\, a. [Cf. F. confidentiel.]
   1. Enjoying, or treated with, confidence; trusted in;
      trustworthy; as, a confidential servant or clerk.
      [1913 Webster]

   2. Communicated in confidence; secret. "Confidential
      messages." --Burke.
      [1913 Webster]

   Confidential communication (Law) See Privileged
      communication, under Privileged.

   Confidential creditors, those whose claims are of such a
      character that they are entitled to be paid before other
      creditors.

   Confidential debts, debts incurred for borrowed money, and
      regarded as having a claim to be paid before other debts.
      --McElrath.
      [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):

confidential
    adj 1: entrusted with private information and the confidence of
           another; "a confidential secretary"
    2: (of information) given in confidence or in secret; "this
       arrangement must be kept confidential"; "their secret
       communications" [syn: confidential, secret]
    3: denoting confidence or intimacy; "a confidential approach";
       "in confidential tone of voice"
    4: the level of official classification for documents next above
       restricted and below secret; available only to persons
       authorized to see documents so classified

Moby Thesaurus II by Grady Ward, 1.0:

24 Moby Thesaurus words for "confidential":
   auricular, between us, chummy, classified, close, closet, esoteric,
   hush-hush, hushed, in petto, inside, intimate, not for publication,
   off the record, private, privileged, sealed, secret, thick, tried,
   trustworthy, trusty, under privilege, unpublishable