in ecclesiastical language, the name of Baptistery a place in which the ceremony of baptism is performed. In the ancient church it was one of the exedrae or buildings distinct from the church itself, and consisted of a porch or anti-room where the persons to be baptized made their confession of faith, and an inner apartment where the ceremony of baptism was performed. Thus it continued till the sixth century, when the baptisteries began to be taken into the church-porch, and afterwards into the church itself. The ancient baptisteries were commonly called baptisteria, photisteria, or places of illumination, because they were the places of instruction preceding baptism; and in these the catechumens seem to have been trained up, and instructed in the first rudiments of the Christian faith.

Those baptisteries were anciently very capacious, because the stated times of baptism returning but seldom, there were usually great multitudes to be baptized at the same time; and also because the manner of baptizing by immersion or dipping under water made it necessary to have a large font. In Venantius Fortunatus it is called aula baptismatis, the large hall of baptism; which was indeed so large, that we sometimes read of councils meeting and sitting therein. This hall, or chapel, was always kept shut during Lent, and the door sealed up with the bishop's seal, not to be opened till Maunday-Thursday. The baptistery was always reputed a sacred place. In the Roman order baptisteries were built of a round figure, and distinguished with the image of St John the Baptist; while over the basin or font was a figure of a dove in gold or silver, to represent the Holy Ghost.

The name baptistery is sometimes also given to a kind of chapel in a large church, which served for the same office. It is an observation of some learned men, that anciently there was but one baptistery in a city, namely, at the bishop's church; and that afterwards they were set up in parish churches, with the special allowance, however, of the bishop.