diapason \dahy-uh-PEY-zuhn, noun:
1. A full, rich outpouring of melodious sound.
2. The compass of a voice or instrument.
3. A fixed standard of pitch.
4. Either of two principal timbres or stops of a pipe organ, one of full, majestic tone (open diapason) and the other of strong, flutelike tone (stopped diapason).
5. Any of several other organ stops.
6. A tuning fork.
During the whole interval in which he had produced those diapason blasts, heard with such inharmonious feelings by the three auditors outside the screen, his thoughts had wandered wider than his notes in conjectures on the character and position of the gentleman seen in Ethelberta’s company.
— Thomas Hardy, The Hand of Ethelberta
And so those two, angry accuser and indifferent accused, faced each other for a moment; while, incessant, dull, might, the thunders of the great cataract mingled with the trembling diapason of the stupendous turbines in the rock-hewn caverns where old Niagara now toiled in fetters, to swell their power and fling gold into their bottomless coffers.
— George Allan England, The Air Trust
Diapason was originally an abbreviation of the Greek phrase “hē dià pāsôn chordôn symphōnía” which meant “the concord through all the notes of the scale.”