A Hot Tub is essentially a bathtub designed to hold several people, and to pump hot, agitated water around them. This is somewhat different from the ancient Roman bath, which could hold several dozen (or even several hundred) bathers in warm water. The modern hot tub is closer to a Japanese bath than any medieval equivalent.
In the SCA, however, hot tubs are not uncommon, especially at post-revel parties or certain very wet events such as Storming of the Gate in Ealdormere. A large number of nobles and peers in a hot tub is sometimes referred to as brass hat soup.