A cold water plunge tub is a type of tub or pool that is designed for cold water therapy. It typically features water that is kept at a very low temperature, often between 50-60 degrees Fahrenheit, and is used for brief periods of time to achieve the benefits of cold water therapy.

Cold water plunge tubs are often used in spa and wellness settings, and are commonly found in facilities that offer hydrotherapy or other types of water-based treatments. They can be used for a variety of purposes, including post-workout recovery, stress relief, and overall health and wellness. Cold water plunges are now popular for the people wanting to use them in their home. 

Some cold water plunge tubs are designed to be used in combination with other types of hydrotherapy, such as hot tubs or saunas, for a full-body wellness experience. Others may be used alone, or as part of a larger cold water immersion program.

It's important to note that cold water therapy, including the use of cold water plunge tubs, should be approached with caution, and should consult the guidance of a healthcare professional to make sure it is right for you. Sudden exposure to cold water can be dangerous for some individuals, especially those with pre-existing medical conditions. It's important to start with short exposure times and gradually increase the duration as your body becomes accustomed to the cold.