diapason
n (Music)
1 either of two stops (open and stopped diapason) usually found throughout the compass of a pipe organ that give it its characteristic tone colour
2 the compass of an instrument or voice
3 (chiefly in French usage)
a a standard pitch used for tuning, esp. the now largely obsolete one of A above middle C = 435 hertz, known as diapason normal (French <PH:d><PH:j><PH:a><PH:p><PH:a><PH:z><PH:P> <PH:n><PH:%><PH:r><PH:m><PH:a><PH:l>)
b a tuning fork or pitch pipe
4 (in classical Greece) an octave
(C14: from Latin: the whole octave, from Greek: (he) dia pason (khordon sumphonia) (concord) through all (the notes), from dia through + pas all)
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diapasonal, diapasonic adj