In Christian architecture the baptistery or baptistry (Old French baptisterie; Latin baptisterium; Greek βαπτιστήριον, 'bathing-place, baptistery', from βαπτίζειν, 'to baptize') is the separate centrally planned structure surrounding the baptismal font. The baptistery may be incorporated within the body of a church or cathedral and be provided with an altar as a chapel. In the early Church, the catechumens were instructed and the sacrament of baptism was administered in the baptistery.(Wikipedia)
"Ceiling of the Baptistry"