An altar is a place of worship, typically a tabletop or other platform to house materials for religious or spiritual rituals.
The word altar is of multiple origins and is found in Old English. It is partly a borrowing from Latin altāre and partly from French alter. For the most part, the spelling remained very similar despite The Great Vowel Shift, palatalization, and standardization of spelling. Occasionally there have been spellings such as allterr that contain additional consonants, but this went away with consonant deletion in Middle English.
Altars are used for a variety of things. Some occult practitioners use altars to celebrate holidays such as the Wheel of the Year, worship non-Christian gods, or to create another type of ritual area. Altars are used in Christian religions for many purposes as well, typically for offerings to the Christian god or the dead.
Sources:
“Altar, N.” Oxford English Dictionary, Oxford UP, December 2023, https://doi.org/10.1093/OED/1067852900.
Personal experience.