An altar is a structure with an upper surface for the presentation of religious offerings, for sacrifices, or for other ritualistic purposes. Altars are found at shrines, temples, churches and other places of worship. They are used particularly in Paganism, Christianity, Buddhism, Hinduism, Judaism (until the destruction of the Second Temple), and Modern Paganism. Many historical faiths also made use of them, including the Roman, Greek, and Norse religions.

Altar in Roskilde Cathedral dwarfed by a huge carved reredos