Advent is a season of the liturgical year observed in most Christian denominations as a time of expectant waiting and preparation for both the celebration of the Nativity of Christ at Christmas and the return of Christ at the Second Coming. Advent is the beginning of the liturgical year in Western Christianity, and is part of the wider Christmas and holiday season.
The name was adopted from Latin adventus “coming; arrival”, translating Greek parousia. In the New Testament, this is the term used for the Second Coming of Christ. Thus, 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 keeping an Advent calendar, lighting an Advent wreath, praying an Advent daily devotional, erecting a Christmas tree or 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 equivalent of Advent in Eastern Christianity is called the Nativity Fast, but it differs in 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 equivalent parousia in its preparatory services. (Source)