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 male swimsuit in North America. They vary in style and design, though most are made of nylon with a mesh lining for quick drying.
In British English, the term trunks is used as in American English as described above, but it can also refer to men's underpants with legs that reach mid thigh, alternatively called boxer briefs.
In American English, the term is sometimes used to refer to trunk hose, defined by the Collins American English Dictionary as "full, baggy breeches reaching about halfway down the thigh, worn in the 16th and 17th cent."