Diapason

1. n. 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.
2. n. The octave, or interval which includes all the tones of the diatonic scale.
3. n. Concord, as of notes an octave apart; harmony.
4. n. The entire compass of tones.
5. n. A standard of pitch; a tuning fork; as, the French normal diapason.

The word meanings were obtained from OPTED(The Online Plain Text English Dictionary), which is based on “The Project Gutenberg Etext of Webster’s Unabridged Dictionary” which is in turn based on the 1913 US Webster’s Unabridged Dictionary (See Project Gutenburg), as a text file.