Ison - Either a symbol name or the supporting part in Byzantine music which provides the "drone" background (see Isokratema). The symbol means the melody should not move but stays on the same pitch as the previous note (the word literally meaning "equal"). Ison can be chanted with the words or on some vowel (preferably "O" or "oo").
Isokratema - The supporting part in Byzantine music which provides the "drone" background. The part is often called "ison" since the pitch of the chanters doesn't change very much (not nearly as much as the melody). The isokratema helps to ground the melody by chanting the base note of the melodic sequence, highlighting the bottom of the melodic contour.
Melody - The main (moving) part of Byzantine music that articulates the text of the hymn through word painting.
Melos - The normal chanting of a hymn with the words, as is done during the Liturgical services.
Parallegē - The chanting of a hymn using the note-names instead of the words. This is analagous to Solfège, in the western world.
Tetrachord - A grouping of four consecutive notes (as in Ni-Pa-Vou-Ga).