In Roman baths, the bathhouse was divided into several sections, each serving a specific function. The cold bath area, where bathers could cool off after using the hot baths, is known as the "frigidarium."
The frigidarium was typically a large, cold pool where bathers would refresh themselves. It was designed to be at a much lower temperature than the hot bath areas like the caldarium, which were heated.
Other areas of the Roman bath included the apodyterium (changing room) and the palaestra (exercise area), but the frigidarium specifically refers to the cold bathing area.