I'm not Catholic or Catholic-lite (Anglican, Lutheran, etc), so I've never really been sure of what period of time this covered. Now I know.
1. a. In the ecclesiastical calendar, the season immediately preceding the festival of the Nativity, now including the four preceding Sundays.
2. The Coming of the Lord Jesus Christ as Saviour of the world; the Incarnation. Hence his expected Second Coming as Judge, and the Coming of the Holy Spirit as at Pentecost.
3. By extension, Any important or epoch-making arrival. In modern usage applied poetically or grandiloquently to any arrival. (This use is unknown to Johnson 1755 and Todd 1818.)
ADDITIONS SERIES 1993
Add: [1.] [b.] Special Combs. Advent calendar, a calendar celebrating the approach of Christmas, esp. one made with flaps or windows opened one each day to reveal a seasonal picture, gift, etc.
Advent candle, a candle lit during Advent; spec. each candle in a ring of four, lit on successive Sundays in Advent to symbolize the coming of light into the world at Christmas (when a fifth central candle completes the group).
Advent ring, a ring of four Advent candles.