What is "hidden octaves"

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

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.

Bring to light the hidden things of darkness.
--1 Cor. iv. 5.

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

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