The Greek term diapason has several meanings in English. One of them is used in music as a synonym for the interval of the octave, dia meaning “over”, pason “all”.
See also diatessaron, hemidiapente and diapente.
(Very) Select Bibliography
Baragwanath, Nicholas. Note-Naming, Galant Schemata, and the “Thread” of Vivaldi’s Concerto for Four Violins, Op. 3 No. 10 (RV 580), in: Music Theory & Analysis 11/1 (2024), 48–77.
Mattheson, Johann. Der vollkommene Capellmeister (Hamburg, 1739).
Walther, Johann Gottfried. Musicalisches Lexicon (Leipzig, 1732).