A Requiem (Latin: rest) or Requiem Mass, also known as Mass for the dead (Latin: Missa pro defunctis) or Mass of the dead (Latin: Missa defunctorum), is a Mass of the Catholic Church offered for the repose of the souls of the deceased, using a particular form of the Roman Missal. It is usually celebrated in the context of a funeral (where in some countries it is often called a Funeral Mass).^{[citation needed]}

Musical settings of the propers of the Requiem Mass are also called Requiems, and the term has subsequently been applied to other musical compositions associated with death, dying, and mourning, even when they lack religious or liturgical relevance.

The term is also used for similar ceremonies outside the Catholic Church, especially in Western Rite Orthodox Christianity, the Anglo-Catholic tradition of Anglicanism, and in certain Lutheran churches. A comparable service, with a wholly different ritual form and texts, exists in the Eastern Orthodox and Eastern Catholic churches as well as some Methodist churches.

The Mass and its settings draw their name from the introit of the liturgy, which begins with the words Requiem aeternam dona eis, Domine (Latin for "Eternal rest grant them, O Lord"), which is cited from 2 Esdras 2:34-35 — requiem is the accusative singular form of the Latin noun requies, "rest, repose". The Roman Missal as revised in 1970 employs this phrase as the first entrance antiphon among the formulas for Masses for the dead, and it remains in use to this day.

In earlier forms of the Roman Rite, some of which are still in use, a Requiem Mass differs in several ways from the usual Mass. Some parts that were of relatively recent origin, including some that have been excluded in the 1970 revision of the regular Mass, are omitted. Examples are the psalm Iudica at the start of Mass, the prayer said by the priest before reading the Gospel (or the blessing of the deacon, if a deacon reads it), and the first of the two prayers of the priest for himself before receiving Communion. Other omissions include the use of incense at the Introit and the Gospel, the kiss of peace, lit candles held by acolytes when a deacon chants the Gospel, and blessings. There is no Gloria in excelsis Deo and no recitation of the Creed; the Alleluia chant before the Gospel is replaced by a Tract, as in Lent; and the Agnus Dei is altered. Ite missa est is replaced with Requiescant in pace (May they rest in peace); the Deo gratias response is replaced with Amen; and the final blessing for the congregation is omitted. Black was the obligatory liturgical colour of the vestments in the earlier forms (including the Missal of 1962), while in the renewed liturgy "the colour black may be used, where it is the practice, in Masses for the Dead". The sequence Dies irae, recited or sung between the Tract and the Gospel, was an obligatory part of the Requiem Mass before the changes as a result of the liturgical reform of the Second Vatican Council. As its opening words Dies irae ("Day of wrath") indicate, this poetic composition speaks of the Day of Judgment in fearsome terms; it then appeals to Jesus for mercy. In the extraordinary form of the Roman Rite, commemorations (i.e. collect, secret, and postcommunion of either lower-ranking liturgical feasts that occur on the same day or votive/seasonal commemorations) are absent from the liturgy; as a result, it is standard practice for a separate, smaller Requiem Missal containing only the rubrics and various Mass formularies for Masses for the dead to be used, rather than the full Missal containing texts that will never be used at Requiems.

In the liturgical reforms of the mid-20th century in the Catholic Church's Roman Rite, there was a significant shift in the funeral rites used by the Church. The theme of sorrow and grief was also made to emphasise the whole community's worship of God in which the deceased is entrusted to God's mercy, based on trust in the salvation value of the Passion, Death and Resurrection of Jesus Christ.

In the 1970s, 1980s and 1990s, the Requiem Mass was sometimes termed a "Mass of the Resurrection" or Mass of Christian Burial, although the former was never official terminology. In the official English ritual, Order of Christian Funerals, published by the Bishops of England and Wales in 1990, the title is given as "Funeral Mass". "Requiem Mass" remains a suitable title for other Masses for the dead and for the Funeral Mass itself (as the proper antiphons remain in force: Introit, "Eternal rest grant ... " / "Requiem æternam dona eis Domine"; Offertory, "Lord Jesus Christ, King of glory, deliver the souls of all the faithful departed ... " / "Domine Iesu Christe, Rex gloriæ, libera animas ..."; Communion, "Let perpetual light shine ..." / "Lux æterna luceat eis, Domine..."). In keeping with those trends of the latter 21st century, the use of white vestments was made an allowable option by the Missal, though only by an indult; black remains the normal colour of all Requiem Masses, including Funeral Masses. Violet, a colour of penance, was also allowed by indult, since penance and reparation for the soul, presumably in Purgatory, is encouraged by the Church. The texts used for the liturgy underwent a similar change, and some of the new options for the readings reinforce an overall theme of Jesus' promise of eternal life.^{[non-primary source needed]}

Requiem is also used to describe any sacred composition that sets to music religious texts which would be appropriate at a funeral, or to describe such compositions for liturgies other than the Roman Catholic Mass. Among the earliest examples of this type are the German settings composed in the 17th century by Heinrich Schütz and Michael Praetorius, whose works are Lutheran adaptations of the Roman Catholic requiem, and which provided inspiration for the German Requiem by Brahms.