'voice, sound') is the name of the eight mode system used for the composition of religious chant in most Christian churches during the Middle Ages. In a modified form, the octoechos is still regarded as the foundation of the tradition of monodic Orthodox chant today (Neobyzantine Octoechos).

The octoechos as a liturgical concept which established an organization of the calendar into eight-week cycles, was the invention of monastic hymnographers at Mar Saba