An altar is a place of worship for one or more deities. The oldest altar was made of a stone slab.
In the Christian sphere, the altar is also referred to as the mensa domini (table of the Lord), following the table of the last supper that Jesus Christ took on the evening of his capture, at the same time the day before his suffering and dying.
As a consequence, the location of the main altar shifted ever further to the wall of the apse, the altar was designated as a high altar, sometimes also as a choral altar. The altar was now no longer free in the room and was so often provided with superstructures – reliefs or altars, so – called retables – on the back. Thus, the artistically richly decorated retable and wing altars of the Gothic and the Baroque were created.
In Catholic churches the main altar is the center of the Eucharistic celebration, as a sign of communion with Christ. In the tradition of the Roman Catholic Church, the altar consecration takes place within the framework of a solemn pontificate. The consecration includes, a. The burial of relics, the consecration prayer and the anointing of the altar with Chrisam. The altar, a sacramental, is described in the Pontificale Romanum and reserved to the bishop.
I have selected a few very nice photos of church altars:
Title pic source: en.wikipedia.org / Saint Peter’s Basilica Altar in Vatican
Pic source: tripideas.org / Kosciol Mariacki, Krakau, Poland
Pic source: revertconvert.blogspot.com / Notre Dame, Montreal, Canada
Pic source: christchurchcambridge.org / Christ Episcopal Church, Cambridge, Maryland
Pic source: flickr.com / Pietà Cathédrale Notre Dame de Paris
Pic source: zoonar.de / St Paul’s Cathedral, Victoria, Australia
Pic source: cas.sk / Altar from Master Paul of Levoca, Spiš, Slovakia
Pic source: : spainisculture.com / Church of San Antonio de los Alemanes, Spain
Pic source: inscrutablebeing.blogspot.com / St. Mary Magdalene in Newark-on-Trent
Pic source: tripadvisor.de / Salvador da Bahia, Brasil