query: Is Byzantine Octoechos a musical system?
passage: Octoechos
Oktōēchos (here transcribed "Octoechos"; Greek: ὁ Ὀκτώηχος Greek pronunciation:[okˈtóixos];[1] from ὀκτώ "eight" and ἦχος "sound, mode" called echos; Slavonic: Осмогласие, Osmoglasie from о́смь "eight" and гласъ, Glagolitic: ⰳⰾⰰⱄⱏ, "voice, sound") is the eight-mode system used for the composition of religious chant in Byzantine, Syriac, Armenian, Georgian, Latin and Slavic churches since the Middle Ages. In a modified form the octoechos is still regarded as the foundation of the tradition of monodic chant in the Byzantine Rite today.
passage: Echos
The noun "echos" in Greek means "sound" in general. It acquired the specialized meaning of "mode" early on in the development of Byzantine music theory since the Octoechos reform in 692.