What's a Swim Trunk ?
Swim trunks are brief shorts, loose-fitting or tight, worn for sports, especially boxing, swimming, and track.
When worn as a swimsuit, trunks are often referred to as swimming trunks or bathing trunks (or with the more general term bathing suit or a synonym) and are normally shorter than board shorts, which extend to the knees. Trunks are the most popular type of men's swimsuit in North America. They vary in style and design, though most are made of nylon with a mesh lining for quick drying.
Source : Swim Trunks Wikipedia