A Christmas carol is a song or hymn associated with Christmas. The difference between a hymn and a carol is not strictly defined; the word “carol” implies a popular quality, and early carols were often danceable. Today the word is almost exclusively applied to Christmas and sometimes Advent hymns. It is correctly applied to Easter hymns as well, but the term "Easter Carol" is not in widespread use.
It is debatable whether secular or not expressly religious songs with a Christmas theme should be called carols. In modern secular use (such as school festivals or public Christmas traditions) such songs are usually included, but not in church services. In the Church of England, carols (except "While shepherds watched") were kept out of actual church services until the 19th century. A carol service introduced at Truro late in that century served as the model for the Festival of Nine Lessons and Carols held at King's College, Cambridge, from 1918 and now broadcast in many countries round the world.