While the definitions of diapason mostly have to do with sound, it amuses me that they're not in particular agreement with one another.
The first listed definition is "a burst of sound" or "the entire range of musical tones." I can see where those might overlap, sure, but hold your hats for part two. The second listing says that diapason refers to "a tuning fork" or "a standard of pitch." It is ALSO used in a literary sense to refer to the "entire compass, range, or scope" of something. OR, if you'd rather, you may have a definition of diapason mean a just octave in Pythagorean tuning, as a treat (from the Latin via the Greek: dia pasōn (khordōn) ‘through all (notes)’.
If that's not enough, then I don't know what will satisfy you! Feel free to use this word in a game of Balderdash or something... Whichever definition suits you at any given time is the right one.
–C