The cooling room of the Roman thermæ, furnished with a cold bath.

an unheated room in the thermae where cold plunge baths were taken after the hot baths.

This was coldest room in the regular sequence of bathing rooms. The room has a variety of designations in original sources (see Nielsen, 1.154, s.v. "Frigidarium: Ancient Sources). Architecturally, the room varied in design and dimensions. The earliest forms are represented in the small, isolated rotundas given over entirely to cold plunge pools found in the baths of Pompeii and Herculaneum. Gradually the room evolved into to the colossal cross-vaulted central chambers of the imperial thermae. The finest example of the latter variety of frigidarium still standing is that of the Baths of Diocletian, known today as the Basilica S. Maria D'Angeli. The modern visitor, entering the basilica on a hot summer's day, can immediately feel the effect of the high roofs and windows -- and most developed frigidaria would have had high roofs exactly for this effect. Occasionally, changing rooms ( apodyteria) doubled as frigidaria, when cold pools were provided in them.