What is the X in X-mas?
late 14c., "petition (to God or a god) for aid or comfort; invocation, prayer;" also "a summoning of evil spirits," from Old French invocacion (12c.), from Latin invocationem (nominative invocatio), noun of action from past participle stem of invocare "to call upon, invoke, appeal to" (see invoke).
a convention of classical literature and of epics in particular, in which an appeal for aid (especially for inspiration) is made to a muse or deity, usually at or near the beginning of the work. Homer's Odyssey, for instance, beginsTell me, Muse, of the man of many ways, who was drivenfar journeys, after he had sacked Troy's sacred citadel.Many were they whose cities he saw, whose minds he learned of,many the pains he suffered in his spirit on the wide sea,struggling for his own life and the homecoming of his companions