thumb|Altar in [[Roskilde Cathedral|Roskilde Lutheran Cathedral beneath a carved reredos]]
An altar is a table or platform for the presentation of religious offerings, for sacrifices, or for other ritualistic purposes. Altars are found at shrines, temples, churches, and other places of worship. They are used particularly in Christianity, Buddhism, Hinduism, and modern paganism. Many historical-medieval faiths also made use of them, including the Roman, Greek, and Norse religions.
Etymology
The modern English word altar was derived from Middle English altar, from Old English alter, taken from Latin altare ("altar"), probably related to adolere ("burn"); thus "burning place", influenced by altus ("high"). It displaced the native Old English word wēofod.
Altars in antiquity
In antiquity, altars were used for making sacrifices to deities; this could include both libations and animal sacrifice.
In Ancient Roman religion, altars were often inscribed with the donor's name and the deity to whom the altar was dedicated. One of the most important surviving Roman altars is the Ara Pacis, dedicated by Augustus Caesar at the beginning of the Pax Romana to the goddess of peace, Pax.<gallery widths="200px" heights="200px" perrow="5" caption="Altars in antiquity">
File:Tel Be'er Sheva Altar 2007041.JPG|Reconstruction of a horned altar at Tel Be'er Sheva, Israel.
File:3217 - Athens - Sto… of Attalus Museum - Kylix - Photo by Giovanni Dall'Orto, Nov 9 2009.jpg|Ancient Greek kylix showing a hoplite offering a sacrifice before an altar, around 480 BC. Ancient Agora Museum of Athens in the Stoa of Attalus
File:Berlin - Pergamonmuseum - Altar 02.jpg|The ancient Altar of Pergamon, reconstructed at the Pergamon museum, Berlin.
File:Opferstein Maria Taferl.jpg|The Opferstein or Sacrifice Rock at Maria Taferl, Austria. It was used by the ancient Celts to make sacrifices upon and is now located in the plaza of the basilica there.
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Judaism
Altars in the Hebrew Bible were typically made of earth or unwrought stone. Altars were generally erected in conspicuous places. The first altar recorded in the Hebrew Bible is that erected by Noah after the flood. Later altars were erected by Abraham, by Isaac, by Jacob, and by Moses.
After the theophany on Mount Sinai, in the Tabernacle, and afterwards in the Temple, only two altars were used: the Altar of Burnt Offering, and the Altar of Incense, both near where the Ark of the Covenant was located.
The remains of three rock-hewn altars were discovered in the Land of Israel: one below Tel Zorah, another at the foot of Sebastia (ancient Samaria), and a third near Shiloh.
Christianity
thumb|left|Dedication of an altar
The word altar, in Greek (see:θυσία), appears twenty-four times in the New Testament. In Catholic and Orthodox Christian theology, the Eucharist is a re-presentation, in the literal sense of the one sacrifice of Christ on the cross being made "present again". Hence, the table upon which the Eucharist is consecrated is called an altar.
The altar plays a central role in the celebration of the Eucharist, which takes place at the altar on which the bread and the wine for consecration are placed. Altars occupy a prominent place in most Christian churches, both Eastern and Western branches. Commonly among these churches, altars are placed for permanent use within designated places of communal worship (often called "sanctuaries"). Less often, though nonetheless notable, altars are set in spaces occupied less regularly, such as outdoors in nature, in cemeteries, in mausoleums/crypts, and family dwellings. Personal altars are those placed in a private bedroom, closet, or other space usually occupied by one person. They are used for practices of piety intended for one person (often referred to as a "private devotion"). They are also found in a minority of Protestant worship places; in Reformed and Anabaptist churches, a table, often called a "Communion table", serves an analogous function.
thumb|A [[home altar in a Methodist Christian household, with a cross and candles surrounded by other religious items]]
The area around the altar is seen as endowed with greater holiness, and is usually physically distinguished from the rest of the church, whether by a permanent structure such as an iconostasis, a rood screen, altar rails, a curtain that can be closed at more solemn moments of the liturgy (as in the Armenian Apostolic Church and Armenian Catholic Church), or simply by the general architectural layout. The altar is often on a higher elevation than the rest of the church.
Churches generally have a single altar, although in the Western branches of Christianity, as a result of the former abandonment of concelebration of Mass, so that priests always celebrated Mass individually, larger churches have had one or more side chapels, each with its own altar. The main altar was also referred to as the "". Since the revival of concelebration in the West, the Roman Missal recommends that in new churches there should be only one altar, "which in the gathering of the faithful will signify the one Christ and the one Eucharist of the Church." This does not exclude altars in distinct side chapels, however, but only separate altars in the main body of the church. But most Western churches of an earlier period, whether Roman Catholic or Anglican, may have a high altar in the main body of the church, with one or more adjoining chapels, each with its own altar, at which the Eucharist may be celebrated on weekdays.
Architecturally, there are two types of altars: Those that are attached to the eastern wall of the chancel, and those that are free-standing and can be walked around, for instance when incensing the altar.
thumb|left|Early [[Coptic Orthodox Church of Alexandria|Coptic altar carved into the wall of the Temple of Isis on the island Philae in Egypt.]]
In the earliest days of the Church, the Eucharist appears to have been celebrated on portable altars set up for the purpose. Some historians hold that, during the persecutions, the Eucharist was celebrated among the tombs in the Catacombs of Rome, using the sarcophagi of martyrs as altars on which to celebrate. Other historians dispute this, but it is thought to be the origin of the tradition of placing relics beneath the altar.
When Christianity was legalized under Constantine the Great and Licinius, formal church buildings were built in great numbers, normally with free-standing altars in the middle of the sanctuary, which in all the earliest churches built in Rome was at the west end of the church. "When Christians in fourth-century Rome could first freely begin to build churches, they customarily located the sanctuary towards the west end of the building in imitation of the sanctuary of the Jerusalem Temple. Although in the days of the Jerusalem Temple the High Priest indeed faced east when sacrificing on Yom Kippur, the sanctuary within which he stood was located at the western end of the Temple. The Christian replication of the layout and the orientation of the Jerusalem Temple helped to dramatize the eschatological meaning attached to the sacrificial death of Jesus the High Priest in the Epistle to the Hebrews." The ministers (bishop, priests, deacons, subdeacons, acolytes), celebrated the Eucharist facing east, towards the entrance. Some hold that for the central part of the celebration the congregation faced the same way. After the sixth century the contrary orientation prevailed, with the entrance to the west and the altar at the east end. Then the ministers and congregation all faced east during the whole celebration; and in Western Europe altars began, in the Middle Ages, to be permanently placed against the east wall of the chancel.
In Western Christian churches
thumb|left|Altar of [[Santa Cecilia in Trastevere with ciborium]]
Most rubrics, even in books of the seventeenth century and later, such as the Pontificale Romanum, continued to envisage the altar as free-standing. The rite of the Dedication of the Church continued to presume that the officiating bishop could circle the altar during the consecration of the church and its altar. Despite this, with the increase in the size and importance of the reredos, most altars were built against the wall or barely separated from it.
In almost all cases, the eastward orientation for prayer was maintained, whether the altar was at the west end of the church, as in all the earliest churches in Rome, in which case the priest celebrating Mass faced the congregation and the church entrance, or whether it was at the east end of the church, in which case the priest faced the eastern apse and had his back to the congregation. This diversity was recognized in the rubrics of the Roman Missal from the 1604 typical edition of Pope Clement VIII to the 1962 edition of Pope John XXIII: ""
When placed close to a wall or touching it, altars were often surmounted by a reredos or altarpiece. If free-standing, they could be placed, as also in Eastern Christianity, within a ciborium (sometimes called a baldachin).
thumb|upright|Altar of [[Newman University Church, Dublin, with an altar ledge occupying the only space between it and the wall]]
The rules regarding the present-day form of the Roman Rite liturgy declare a free-standing main altar to be "desirable wherever possible". Similarly, in the Anglican Communion, the rubrics of the Book of Common Prayer assumed an altar fixed against the wall, until Prayer Book revision in the twentieth century removed language which assumed any particular form of altar.
As well as altars in the structural sense, it became customary in the West to have what in Latin were referred to as (portable altars), more commonly referred to in English as altar stones. When travelling, a priest could take one with him and place it on an ordinary table for saying Mass. They were also inserted into the centre of structural altars especially those made of wood. In that case, it was the altar stone that was considered liturgically to be the altar. The Pontificale Romanum contained a rite for blessing at the same time several of these altar stones. In the East the antimension served and continues to serve the same purpose.
The term movable altar or portable altar is now used of a full-scale structural altar, with or without an inserted altar stone, that can be moved.
Movable altars include the free-standing wooden tables without altar stone, placed in the choir away from the east wall, favoured by churches in the Reformed tradition. Altars that not only can be moved but are repeatedly moved are found in low church traditions that do not focus worship on the Eucharist, celebrating it rarely. Both Catholics and Protestants celebrate the Eucharist at such altars outside of churches and chapels, as outdoors or in an auditorium.
Catholic Church
thumb|left|High altar of [[Saint Peter's Basilica, Rome]]
The Eastern Catholic Churches each follow their own traditions, which in general correspond to those of similar Eastern Orthodox or Oriental Orthodox Churches. All Christian Churches see the altar on which the Eucharist is offered as the "table of the Lord" () mentioned by Saint Paul. The rules indicated here are those of the Latin Church.
The Latin Church distinguishes between fixed altars (those attached to the floor) and movable altars (those that can be displaced), and states: "It is desirable that in every church there be a fixed altar, since this more clearly and permanently signifies Christ Jesus, the Living Stone. In other places set aside for sacred celebrations, the altar may be movable."
The liturgical norms state:
:It is fitting that the tradition of the Roman liturgy should be preserved of placing relics of martyrs or other saints beneath the altar. However, the following should be noted:
:(a) Relics intended for deposition should be large enough that they can be recognized as parts of human bodies. Hence excessively small relics of one or more saints must not be deposited.
:(b) The greatest care must be taken to determine whether relics intended for deposition are authentic. It is better for an altar to be dedicated without relics than to have relics of doubtful credibility placed beneath it.
:(c) A reliquary must not be placed on the altar, or in the table of the altar; it must be beneath the table of the altar, as the design of the altar may allow.