The word "Christmas" is derived from the Old English word "Cristes-messe", which means "Christ's Mass". It is a combination of two words: "Cristes", which is a form of the name "Christ", and "messe", which means "mass". The word "mass" refers to the religious service that is held to commemorate the birth of Jesus Christ. Therefore, the word "Christmas" refers to the celebration of the birth of Jesus Christ.