An altar is any structure upon which offerings such as sacrifices are made for religious purposes. Altars are usually found at shrines, and they can be located in temples, churches and other places of worship. Today they are used particularly in Christianity, Buddhism, Hinduism, Shinto, Taoism, as well as in Neopaganism and Ceremonial Magic. Judaism did so until the destruction of the Second Temple. Many historical faiths also made use of them, including Greek and Norse religion.

Altars (Hebrew: מזבח‎, mizbe'ah, "a place of slaughter or sacrifice") in the Hebrew Bible were typically made of earth or unwrought stone. Altars were generally erected in conspicuous places. The first altar recorded in the Hebrew Bible is that erected by Noah . Altars were erected by Abraham, by Isaac, by Jacob, and by Moses, (Adonai-nissi).

After the theophany on Mount Sinai, in the Tabernacle—and afterwards in the Temple—only two altars were used: the Altar of Burnt Offering, and the Altar of Incense.

The word "altar", in Greek θυσιαστήριον (see:θυσία), appears twenty-four times in the New Testament. Significantly, Hebrews 13:10 shows Christians having an altar of which those who did not believe in Jesus could not partake, a reference to the eternal, once-for-all sacrifice of Jesus Christ, thus fulfilling the sacrificial laws of the Old Testament. In Catholic and Orthodox Christian theology, the Eucharist is a re-presentation, in the literal sense of the one sacrifice being made "present again". Hence, the table upon which the Eucharistic meal (the Bread and the Wine) is eaten is called an altar.