In Roman Catholic churches the sanctuary light is the hanging light that shines constantly before the altar, where the reserved sacrament is kept in the tabernacle (§2). The purpose of the lamp is “to indicate and honor the presence of Christ” (1983 CIC 940).
Evidence exists of the use of this light in the West from the 11th and 12th centuries. The Rituale Romanum (1614) made it obligatory. Oil or wax is usually burned, but electric light is permitted.