Swim trunks
Swim trunks, also known as swimming trunks, are a form of swimsuit worn by men and boys specifically for swimming. As such, they are typically made of materials designed for being wet and remaining comfortable and hydrodynamic to not impede the swimmer. The choice of materials makes them distinct from underwear which may have a similar shape. The term is a catch-all for a number of styles of garment such as briefs (very short with no leg coverage), shorts (fabric extends beyond the crotch and slightly onto the legs) and "jammers" (extend further down the legs, to mid thigh or even the knee, and popular with competitive swimmers).
Especially in North America, the term swim trunks can include water-friendly variants of articles like beach shorts and boardshorts which are typically baggy, casual, and more modest, but also impede the performance of the swimmer.