Tone is the most common English translation of the Greek word ηχος (ichos), referring traditionally to one of the eight modes of the monophonic chant traditional and canonical in the Orthodox Church. There is only one set of eight tones used for all such music, though some of the tones have particular special variants.
In the newer, harmonized, non-modal, polyphonic forms of Church music which have become characteristic especially of the Church of Russia and some of the churches birthed or influenced by it, tone refers to formulae of melodies and chord progressions. In this tradition, there are multiple sets of tones used for different hymns (e.g., "canon tone," "akathist tone," etc.), multiplying even more where regional variants become used beyond their respective regions.
A number of experts in the traditional modal forms of Orthodox music prefer that ηχος be translated as mode rather than tone, since mode in English refers more properly to the same concept, while tone is ambiguous in English, has no prior usage for this concept, and is closer to the Greek τονος (tonos), which has another meaning.