An altar is actually called "the altar", although if there are many altars in a Church, they may be called by a specific name, like the main altar, the high altar, the side altar, the Blessed Virgin Mary altar, the altar of repose, the Blessed Sacrament altar, the St. Joseph altar, etc.

Altar.

It is a cloth placed on an altar.

The hanging cloth on an altar is called an altar cloth or an altar frontal. It is used to decorate the altar and often changes according to the liturgical season or special occasions in the church.

the altar is used for prayer and sacrifice