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]