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;
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
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.
Bring to light the hidden things of darkness.
--1 Cor. iv.
My heart, which by a secret harmony
Still moves with thine, joined in connection
By what best way,
Whether of open war, or covert guile,
We now debate. --Milton.