A side-altar or bye-altar is an altar that is subordinate to the central or high altar in a church. The term is generally applied to altars that are situated in the bay or bays of the nave, transepts, etc.[1] Side-altars may be recessed in a side-chapel, or against a main aisle wall.
This article incorporates text from a publication now in the public domain: Herbermann, Charles, ed (1913). Catholic Encyclopedia. Robert Appleton Company.