To an organist, “diapason” refers to the tonal sounding of the flue pipes — basically all of the notes possible. The word comes from the Greek phrase “dia pasōn (khordōn),” which means “through all (notes).” In other musical uses, “diapason” can refer to any octave (not exclusive to the organ), and also a great swelling of harmonic sound. Away from music, yet still in a poetic sense, “diapason” can mean the entire range or scope, such as “the diapason of possibilities was brimming before me.”