An antependium (from Latin ante- and pendēre "to hang before"; pl: antependia), also known as a parament or hanging, or, when speaking specifically of the hanging for the altar, an altar frontal (Latin: pallium altaris), is a decorative piece, usually of textile, but also metalwork, stone or other material that can adorn a Christian altar.
Specifically, and as the etymology of the word suggests, an antependium hangs down in front of whatever it covers, and is to be distinguished from the altar linens which are used in the service of the Eucharist, and an altar cloth which covers the top of the altar table (mensa).
"Antependium" is the word used for elaborate fixed altar frontals, which, in large churches and especially in the Ottonian art of the Early Medieval period, were sometimes of gold studded with gems, enamels and ivories, and in other periods and churches often carved stone, painted wood panel, stucco, or other materials, such as azulejo tiling in Portugal.
When the front of an altar is elaborately carved or painted, the additional cloth altar frontal normally reaches down only a few inches from the top of the altar table; this is called a "frontlet". In other cases it may reach to the floor (the "frontal", properly so called). In both situations, it will usually cover the entire width of the altar. A "Jacobean frontal" will cover the entire altar, reaching down to the floor on all four sides.
The Anglican Canons of 1603 order that the Lord's Table should be "covered, in time of Divine Service, with a carpet of silk or other decent stuff, thought meet by the Ordinary of the place" (can. 82).
Covers for lecterns and pulpits are generally similar to a frontlet, normally covering the "desk" of the lectern or pulpit and handing down about a foot or longer in front (visible from the congregation).
In the Orthodox Church, the Holy Table (altar) may be covered with one or two coverings. There is always an outer frontal, covering the top of the Holy Table and hanging down several inches on all four sides. This kind is used alone if the front of the Holy Table is elaborately carved or decorated. For a "fully vested" Holy Table, a second, inner hanging is used. This covers the Holy Table fully on the top and hangs down to the floor on all four sides.