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 situated in bays of the nave, transepts, etc.^[1] Side-altars may be recessed in a side-chapel, or simply built against a main aisle wall.

Before the liturgical reforms arising from the Second Vatican Council, separate Masses may be celebrated simultaneously by other priests at side-altars, even as there is an ongoing Mass at the high altar.

• Lady chapel

 This article incorporates text from a publication now in the public domain: Herbermann, Charles, ed. (1913). "Altar Side". Catholic Encyclopedia. New York: Robert Appleton Company.