Elevated platforms or constructions upon which religious sacrifice is offered or other religious or spiritual activities carried out. In a Christian church altars are platforms where Eucharistic elements are consecrated, in the ancient tradition of a sacrifice. Includes both indoor small, tablelike fixtures and larger, free-standing, outdoor structures. It consists of a horizontal part, the mensa, placed on various types of supports. For the surface at which communion is celebrated in Protestant churches, use "communion tables."

Source: AAT 

http://vocab.getty.edu/page/aat/300003725