The Collaborative International Dictionary of English v.0.48:
Diapason \Di`a*pa"son\, n. [L., fr. Gr. diapasw^n (i. e., "h
dia` pasw^n chordw^n symfoni`a the concord of the first and
last notes, the octave); dia` through + pasw^n, gen. pl. of
pa^s all: cf. F. diapason. Cf. Panacea.]
1. (Gr. Mus.) The octave, or interval which includes all the
tones of the diatonic scale. Compare disdiapason.
2. Concord, as of notes an octave apart; harmony.
The fair music that all creatures made . . .
In perfect diapason. --Milton.
3. The entire compass of tones; the entire compass of tones
of a voice or an instrument.
Through all the compass of the notes it ran,
The diapason closing full in man. --Dryden.
4. A standard of pitch; a tuning fork; as, the French normal
5. 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.