The antimension (Greek for "instead of the table") is one of the furnishings of the altar. It is a rectangular piece of cloth, of linen or silk, with representations of the entombment of Christ, the four Evangelists, and scriptural passages related to the Eucharist. It often has a very small relic sewn into it. It is unfolded on the altar before the Anaphora, and the Eucharist is consecrated on it. The antimension must be consecrated and signed by the bishop, indicating his permission for the Eucharist to be celebrated in his absence. It is, in effect, the priest's permission to officiate.
The antimension is a substitute altar. A priest may celebrate the Eucharist on the antimension even if there is no properly consecrated altar. In emergencies, war and persecution, the antimension serves a very important pastoral need. The eileton is now often used to wrap the antimension when it is not in use.