What is the X in X-mas?
c.1500, "a versicle sung responsively," from Middle French antiphone "hymn" or directly from Medieval Latin antiphona, from Greek antiphona, from anti- "over against" (see anti-) + phone "voice" (see fame (n.)). A re-adoption of the word which had become anthem in English and lost its original meaning.
in Roman Catholic liturgical music, chant melody and text sung before and after a psalm verse, originally by alternating choirs (antiphonal singing). The antiphonal singing of psalms was adopted from Hebrew worship by the early Christian churches, notably that of Syria, and was introduced into the West in the 4th century by St. Ambrose. The two choirs both sang the psalm text or, alternatively, one choir sang a short refrain between the psalm verses (V) sung by the other choir. The refrain was called an antiphon (A). The resulting musical form was A V1 A V2 A. Actually, most of the presentations of the antiphon were in abbreviated form. The antiphon text normally referred to the meaning of the feast day or the psalm. Canticles from the New or Old Testament might also be sung in this way