What is the X in X-mas?
late Old English albe, from Late Latin alba (in tunica alba or vestis alba "white vestment"), fem. of albus "white," from PIE root *albho- "white" (cf. Greek alphos "white leprosy," alphiton "barley meal;" Old High German albiz, Old English elfet "swan," literally "the white bird;" Old Church Slavonic and Russian lebedi, Polish łabędź "swan;" Hittite alpash "cloud").
liturgical vestment worn in some services by Roman Catholic officiants, some Anglicans, and some Lutherans. A symbol of purity, it is a full-length, long-sleeved, usually white linen tunic secured at the waist by a cord or belt called a cincture. The equivalent vestment in the Eastern churches is the sticharion.