Baptisterium

The baptisterium in the frigidarium of the thermes at Pompeii.

In classical antiquity, a baptisterium (Ancient Greek: βαπτιστήριον) was a large basin installed in private or public baths into which bathers could plunge, or even swim about. It is more commonly called natatorium or piscina.

References

•  This article incorporates text from a publication now in the public domain: Peck, Harry Thurston, ed. (1898). "Baptisterium". Harpers Dictionary of Classical Antiquities. New York: Harper & Brothers.