(1):

(n.) The octave, or interval which includes all the tones of the diatonic scale.

(2):

(n.) Concord, as of notes an octave apart; harmony.

(3):

(n.) The entire compass of tones.

(4):

(n.) One of certain stops in the organ, so called because they extend through the scale of the instrument. They are of several kinds, as open diapason, stopped diapason, double diapason, and the like.

(5):

(n.) A standard of pitch; a tuning fork; as, the French normal diapason.