Structure upon which offerings such as sacrifices are made for religious purposes
An altar is a table or platform 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 Christianity, Buddhism, Hinduism, and modern paganism. Many historical-medieval faiths also made use of them, including the Roman, Greek, and Norse religions. Etymology.
https://en.wikipedia.org/wiki/Altar