What is "open diapason"

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 diapason.

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. [1913 Webster] ||

n. (context music English) A certain stop in an organ, in which the pipes or tubes are formed like the mouthpiece of a flageolet at the end where the wind enters, and are open at the other end.