Originally the term baptisterium denoted the basin in the frigidarium (cold bath) of the Roman baths. From the fourth century it came into use for the piscina (Lat. for “tank, basin”), or baptismal church. Other names were balneus and loutron (bath) and, among Christians, phōtistērion (enlightenment). From the third century certain cultic places were set apart for the purpose of baptism. In keeping with the form and situation of the piscina, these were often round or octagonal with surrounding pillars, upper lighting, and a cupola. Such forms derived from the a…