The Collaborative International Dictionary of English v.0.48:

Hidden \Hid"den\, p. p. & a.
   from Hide. Concealed; put out of view; secret; not known;
   mysterious.
   [1913 Webster]

   Hidden fifths or Hidden octaves (Mus.), consecutive
      fifths or octaves, not sounded, but suggested or implied
      in the parallel motion of two parts towards a fifth or an
      octave.

   Syn: Hidden, Secret, Covert.

   Usage: Hidden may denote either known to on one; as, a hidden
          disease; or intentionally concealed; as, a hidden
          purpose of revenge. Secret denotes that the thing is
          known only to the party or parties concerned; as, a
          secret conspiracy. Covert literally denotes what is
          not open or avowed; as, a covert plan; but is often
          applied to what we mean shall be understood, without
          openly expressing it; as, a covert allusion. Secret is
          opposed to known, and hidden to revealed.
          [1913 Webster]

                Bring to light the hidden things of darkness.
                                                  --1 Cor. iv.
                                                  5.
          [1913 Webster]

                My heart, which by a secret harmony
                Still moves with thine, joined in connection
                sweet.                            --Milton.
          [1913 Webster]

                By what best way,
                Whether of open war, or covert guile,
                We now debate.                    --Milton.
          [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Hide \Hide\ (h[imac]d), v. t. [imp. Hid (h[i^]d); p. p.
   Hidden (h[i^]d"d'n), Hid; p. pr. & vb. n. Hiding
   (h[imac]d"[i^]ng).] [OE. hiden, huden, AS. h[=y]dan; akin to
   Gr. key`qein, and prob. to E. house, hut, and perh. to E.
   hide of an animal, and to hoard. Cf. Hoard.]
   1. To conceal, or withdraw from sight; to put out of view; to
      secrete.
      [1913 Webster]

            A city that is set on an hill can not be hid.
                                                  --Matt. v. 15.
      [1913 Webster]

            If circumstances lead me, I will find
            Where truth is hid.                   --Shak.
      [1913 Webster]

   2. To withhold from knowledge; to keep secret; to refrain
      from avowing or confessing.
      [1913 Webster]

            Heaven from all creatures hides the book of fate.
                                                  --Pope.
      [1913 Webster]

   3. To remove from danger; to shelter.
      [1913 Webster]

            In the time of trouble he shall hide me in his
            pavilion.                             --Ps. xxvi. 5.
      [1913 Webster]

   To hide one's self, to put one's self in a condition to be
      safe; to secure protection. "A prudent man foreseeth the
      evil, and hideth himself." --Prov. xxii. 3.

   To hide the face, to withdraw favor. "Thou didst hide thy
      face, and I was troubled." --Ps. xxx. 7.

   To hide the face from.
      (a) To overlook; to pardon. "Hide thy face from my sins."
          --Ps. li. 9.
      (b) To withdraw favor from; to be displeased with.

   Syn: To conceal; secrete; disguise; dissemble; screen; cloak;
        mask; veil. See Conceal.
        [1913 Webster]

WordNet (r) 3.0 (2006):

hidden
    adj 1: not accessible to view; "concealed (or hidden) damage";
           "in stormy weather the stars are out of sight" [syn:
           concealed, hidden, out of sight]
    2: designed to elude detection; "a hidden room or place of
       concealment such as a priest hole"; "a secret passage"; "the
       secret compartment in the desk" [syn: hidden, secret]
    3: difficult to find; "hidden valleys"; "a hidden cave"; "an
       obscure retreat" [syn: hidden, obscure]

Moby Thesaurus II by Grady Ward, 1.0:

114 Moby Thesaurus words for "hidden":
   abstract, abstruse, arcane, beclouded, behind the curtain,
   behind the scenes, between the lines, blind, buried, cabalistic,
   camouflaged, censored, classified, close, closed, clouded,
   concealed, covered, covert, cryptic, dark, deep, delitescent,
   disguised, dormant, eclipsed, enigmatic, esoteric, guarded,
   hermetic, hibernating, hid, hush-hush, imperceptible, in a cloud,
   in a fog, in eclipse, in purdah, in the wings, incommunicado,
   indiscernible, insensible, invisible, latent, lurking, muffled,
   mysterious, mystic, obfuscated, obscure, obscured, occult,
   out of sight, possible, potential, private, privy, profound,
   recondite, restricted, secluded, secluse, secret, sequestered,
   shrouded, sightless, sleeping, smothered, stifled, submerged,
   suppressed, top secret, transcendental, ulterior, unapparent,
   unbeheld, unbeholdable, unbreatheable, under an eclipse,
   under cover, under house arrest, under security, under the surface,
   under wraps, underground, underlying, undiscernible, undisclosable,
   undisclosed, undivulgable, undivulged, unknown, unmanifested,
   unnoticed, unobserved, unperceivable, unperceived, unrealized,
   unrevealable, unrevealed, unseeable, unseen, unspoken, untellable,
   untold, unutterable, unuttered, unviewed, unwhisperable,
   unwitnessed, veiled, viewless, virtual, wrapped in clouds