Swim trunks, also known as swimming trunks, are a form of swimsuit - clothing worn specifically for swimming. As such, they are usually made of materials specifically 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 - 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 also include water-friendly variants of such articles as beach shorts and boardshorts which are typically baggy, casual, and more modest—but which impede the performance of the swimmer.