Noun
echoi (plural echois)
(music) One of a set of eight modes, octoechoi, of music in Byzantine music. It selects the chant style or genre.
It is generally assumed that Carolingian theorists imported monastic Octoechos propagated in the patriarchates of Jerusalem ( Mar Saba ) and Constantinople ( Stoudios Monastery ) which meant the eight echoi they used for the composition of hymns (e. Source: Internet
This system is called oktoechos and is also divided into eight categories, called echoi. Source: Internet