Epiphany

From Old French epiphanie, from Late Latin epiphania, from Greek epiphaneia, "appearance, manifestation", used of a god showing himself to mortals, from epiphainein, formed from epi ("upon") and phainein ("to show, to bring to light"). In English since the 14th century as the name of the Christian feast of 6 January, which marks the showing of Christ to the Magi. The general sense of a revelation is found from the 17th century, and the sense of a sudden insight owes much to James Joyce, who used the word for a moment in which the nature of a thing shows itself.

With a capital letter, Epiphany is the feast of 6 January. In lower case it is the sudden insight, often in the phrase "to have an epiphany".