An undersuit is a garment worn under a diver's drysuit or under a spacesuit.
Undersuits provide an extra layer of insulation to that provided by the outer shell of a suit. An undersuit is normally a one-piece garment of a similar shape to a boiler suit. Entry is normally via a zip which runs from the neck down to the waist area.
Undersuits for diving can vary in thickness depending on the water temperature and what type of drysuit they are used under. Neoprene drysuits have more in-built insulation than membrane drysuits so require less insulation from an undersuit, although most of the suit insulation disappears as the neoprene compresses at depth. Diving undersuits are often made from a several different materials layered on top of each other. An example of the material construction might be an outer windproof shell followed by a layer of synthetic insulation (e.g. Thinsulate), then an inner comfort layer such as fleece.
Drysuits used for watersports tend to require thinner undersuits than those used for diving and are commonly made from fleece material.