The antimension, (from the Greek: "instead of the table"), is among the most important furnishings of the altar in Orthodox Christian liturgical traditions.
In the first centuries of Christianity, the Divine Liturgy was celebrated on the tombs of the Martyrs and this was celebrated by the Bishop. Later, as the Church expanded and the size of a typical Diocese with it, the Bishops of the early Church began to ordain Priests as their representatives to the growing number of Christian communities. Only with the Bishop's permission could a community and its Priest serve the Liturgy and the same holds true today. One of the vehicles by which these important ancient practices are effected today is a simple piece of cloth, folded within another, and resting always on the Holy Table of every Orthodox church the Antimension.
The Antimension is a rectangular piece of cloth, gold in color, measuring about 18 by 24 inches, and while on the Holy Table it is folded within another cloth, red in color, called the Iliton, which represents the swaddling clothes and the burial shroud of Jesus Christ. Depicted on the top of the Antimension is an Icon of the Burial of Christ, along with Icons of the four Evangelists, as well as Saints Basil the Great and John Chrysostom, for whom the usual Divine Liturgies are named. Sewn into every Antimension is an incorruptible relic of a Saint, making real the early liturgical connection with the Martyrs who died rather than renounce Christ, and whose blood, after the Blood of Christ, formed the very foundation of the Church.
Printed on every Antimension are the words: By the grace of the All-Holy, Life-giving Spirit, this Antimension, the Holy Table, is consecrated for the Offering on it of the Body and Blood of our Lord in the Divine Liturgy. Each one is signed by the ruling Bishop of the Diocese and placed on the Holy Table, constituting his permission for the community to exist as an Orthodox parish and to celebrate the Liturgy. This is so, since true Christianity has always held that without the Bishop there is no Church and through the Bishop comes our unity of Faith and Communion which is Orthodoxy.
The word Antimension is a combination of Greek and Latin which means in place of the table. While Holy Tables were always to have been consecrated and relics placed inside of them, it was not always possible for the Bishop to visit each community to do so. For that reason, Bishops consecrated cloths or boards and sent them to each community to be used in place of the consecrated Holy Table. This also allowed for portable Holy Tables for travelers. The use of the Antimension is mandatory, even on Holy Tables which have been consecrated, and a Priest is not permitted to celebrate the Divine Liturgy without it. Military Chaplains and Missionaries also use it instead of the table when serving in remote areas.
From "These Truths We Hold - The Holy Orthodox Church: Her Life and Teachings". Compiled and Edited by A Monk of St. Tikhon's Monastery. Copyright 1986 by the St. Tikhon's Seminary Press, South Canaan, Pennsylvania 18459.