What is the X in X-mas?
"mass for repose of the soul of the dead," c.1300, from Latin requiem, accusative singular of requies "rest (after labor), repose," from re-, intensive prefix (see re-), + quies "quiet" (see quiet (adj.)). It is the first word of the Mass for the Dead in the Latin liturgy: Requiem æternam dona eis, Domine .... ["Grant them eternal rest, O Lord ...."]
In music, a Mass for one or more dead persons, containing biblical passages and prayers for the admission of the dead to heaven. The term has been loosely applied to other musical compositions in honor of the dead. A German Requiem by Johannes Brahms, for example, uses texts from the Bible but is not a Mass.