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.
      [1913 Webster]

   2. Concord, as of notes an octave apart; harmony.
      [1913 Webster]

            The fair music that all creatures made . . .
            In perfect diapason.                  --Milton.
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   3. The entire compass of tones; the entire compass of tones
      of a voice or an instrument.
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            Through all the compass of the notes it ran,
            The diapason closing full in man.     --Dryden.
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   4. A standard of pitch; a tuning fork; as, the French normal
      diapason.
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   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]

WordNet (r) 3.0 (2006):

diapason
    n 1: either of the two main stops on a pipe organ [syn:
         diapason, diapason stop]

Moby Thesaurus II by Grady Ward, 1.0:

155 Moby Thesaurus words for "diapason":
   English horn, accord, accordance, air, attune, attunement, bassoon,
   baton, block flute, bombard, bourdon, carry, cello, chime, chiming,
   chromatic scale, claribel, clarinet, clarion, compass, concentus,
   concert, concert flute, concord, concordance,
   consecutive intervals, consonance, consonancy, consort, cornet,
   cornopean, cromorna, cymbel, degree, descant, diatessaron,
   diatonic interval, diatonic semitone, dodecuple scale, dulciana,
   enharmonic diesis, enharmonic interval, enharmonic scale, euphony,
   fifth, flute stop, foundation stop, fourniture, fourth, gamba,
   gamut, gedeckt, gemshorn, great scale, half step, halftone,
   harmonic flute, harmonics, harmony, heavy harmony, homophony,
   hybrid stop, interval, koppel flute, larigot, lay, less semitone,
   major scale, measure, melodia, melodic interval, melodic minor,
   metronome, minor scale, mixture, monochord, monody, music stand,
   mutation stop, mute, nazard, note, oboe, octave, octave scale,
   organ stop, parallel octaves, pentatonic scale, piccolo,
   pitch pipe, plein jeu, posaune, principal, quint, quintaten,
   radius, range, rank, ranket, reach, reed stop, register,
   rhythmometer, rohr flute, scale, scope, second, semitone,
   sesquialtera, seventh, shawm, sixth, sonometer, spectrum,
   spitz flute, step, stick, stop, stopped diapason, stopped flute,
   strain, stretch, string diapason, string stop, sweep, symphony,
   synchronism, synchronization, temperament, third,
   three-part harmony, tierce, tone, tone measurer, tremolo, trombone,
   trumpet, tune, tuning, tuning bar, tuning fork, tuning pipe,
   twelfth, unda maris, unison, unison interval, unisonance, vibrato,
   viola, voix celeste, vox angelica, vox humana, warble, whole step,
   whole-tone scale