On Diaphony

Musica Enchiriadis (ca. 900), trans. 31
"...the same guiding principle that controls the concord of pitches regulates the natures of mortals. Through these numerical relationships, by which unlike sounds concord with each other, the eternal harmony of life and of the conflicting elements of the whole world is united as one with material things"

Scolica Enchiriadis (ca. 900), trans. 65
"Harmony is deemed the agreeable mixture of different pitches; music is the theory [ratio] of that agreeableness. Just as music is bound up in every respect with this theory of numbers, as are also the other mathematical disciplines, so it is proper that they be understood through numbers."

Guido, Micrologus (1026-28), Chapter 18 (trans., 77)
"Diaphony sounds as a separateness of [simultaneous] sounds, which we also call organum, in which notes distinct from each other make dissonance harmoniously and harmonize in their dissonance."

John (ca. 1100), trans. 159
Diaphony is the sounding of different but harmonious notes, which is carried on by at least two singers, so that while one holds to the original melody, another may range aptly among other tones, and at each breathing point both may come together on the same note or at the octave. This method of singing is popularly called "organum"....Diaphony means "twofold sound" or "difference of sound."

Montpellier Organum Treatise (early 1100s)
Diaphony is two-part song; that is its definition.