"Diapason" is a word in ENGLISH

A standard of pitch; a tuning fork; as, the French normal
diapason.

The entire compass of tones.

Concord, as of notes an octave apart; harmony.

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.

The octave, or interval which includes all the tones of
the diatonic scale.