The word means a manifestation or ‘revelation’ and is similar to ‘theophany’ or revelation of the divine. There are several instances of this in the OT, notably God's revelation to Moses at the Burning Bush (Exod. 3: 2). For Christians the supreme manifestation of God was in the incarnation and two feasts have commemorated it in the calendars of Western and Eastern Christendom.
From the middle of the 5th cent. CE the Church at Rome commemorated the revelation of Christ to the Gentiles as signified by the visit to the infant Jesus of the Magi (Matt. 2: 11) on 6 January. And from Rome this observance spread in the West, with Christmas Day on 25 December for the birth of Jesus, established not later than 336.
It was not always so. From the 3rd cent. in the East the Epiphany on 6 January had commemorated not only the birth, including the visit of the Magi, but also the baptism of Jesus and even his first miracle at Cana (John 2: 1–11). By the end of the 4th cent. Epiphany concentrated on the baptism of Jesus, while 25 December was borrowed from the West for Jesus' birth.