Source : Webster's Revised Unabridged Dictionary (1913)

Diapason \Di`a*pa"son\, n. [L., fr. Gr. ? (i. e., ? ? ? the
   concord of the first and last notes, the octave); dia`
   through + ?, gen. pl. of ? all: cf. F. diapason. Cf.
   {Panacea}.]
   1. (Gr. Mus.) The octave, or interval which includes all the
      tones of the diatonic scale.

   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.

            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.

Source : WordNet®

diapason
     n : either of the two main stops on a pipe organ [syn: {diapason
         stop}]