Advent is a season observed in most Christian denominations as a time of waiting and preparation for both the celebration of Jesus's birth at Christmas and the return of Christ at the Second Coming. It begins on the fourth Sunday before Christmas, often referred to as Advent Sunday. Advent is the beginning of the liturgical year in Western Christianity. The name comes from Latin adventus ('coming; arrival'), translating the Greek parousia from the New Testament, originally referring to the Second Coming.

The season of Advent in the Christian calendar anticipates the "coming of Christ" from three different perspectives: the physical nativity in Bethlehem, the reception of Christ in the heart of the believer, and the eschatological Second Coming.

Practices associated with Advent include Advent calendars, lighting an Advent wreath, praying an Advent daily devotional, erecting a Chrismon tree, lighting a Christingle, as well as other ways of preparing for Christmas, such as setting up Christmas decorations, a custom that is sometimes done liturgically through a hanging of the greens ceremony.

The analogue of Advent in Eastern Christianity is called the Nativity Fast, but it differs in meaning, length, and observances, and does not begin the liturgical church year as it does in the West. The Eastern Nativity Fast does not use the term parousia in its preparatory services.

In the Anglican, Lutheran, Moravian, Presbyterian, and Methodist calendars, Advent begins on the fourth Sunday before Christmas (the Sunday that falls on or closest to 30 November, always between 27 November and 3 December; it is the Sunday between the last Thursday of November and the first Thursday of December), and ends by Christmas Eve on 24 December. In the Roman Rite of the Catholic Church, Advent begins with the First Vespers of the First Sunday of Advent and ends with the Deus, in adiutorium of the First Vespers of Christmas. The first day of Advent also begins a new liturgical year.

In the Ambrosian Rite and the Mozarabic Rite of the Catholic Church, Advent begins on the sixth Sunday before Christmas (the Sunday that falls on or closest to 16 November, always between 13 November and 19 November; it is the Sunday before the third Tuesday of November).

For Western Christians of the Catholic, Lutheran and Anglican traditions, Advent signifies preparation for a threefold coming of Christ: firstly in the Incarnation at Bethlehem, then in a perpetual sacramental presence in the Eucharist, and thirdly at his Second Coming and final judgement. Furthermore, Advent is a time to focus on his present coming to mankind in the Word and Sacraments.

It is not known when the period of preparation for Christmas that is now called Advent began, though it was certainly in existence from about 480; the novelty introduced by the Council of Tours of 567 was to order monks to fast every day in the month of December until Christmas. According to J. Neil Alexander, it is "impossible to claim with confidence a credible explanation of the origin of Advent".