Underwear, underclothing, or undergarments are items of clothing worn beneath outerwear, usually in direct touch with the skin, however, they may consist of more than one layer. They protect outer clothing from being soiled or destroyed by biological excretions, reduce friction between outerwear and skin, contour the body, and give hiding or support for sections of it. Long underwear is sometimes worn in cold conditions to give extra warmth. The religious significance is attached to some types of undergarments. Some clothing items are intended to be worn as underwear only, whereas others, such as T-shirts and certain types of shorts, can be worn as both underwear and outerwear. Some underwear can be used as nightwear or swimsuits if constructed of the right material or materials, while others are designed for sexual attraction or visual appeal.

Underwear is normally divided into two types: those used to cover the torso and those worn to cover the waist and legs, however, some garments cover both. Females and men typically wear different types of underwear. Females often wear bras and panties (knickers in British English), whereas men typically wear classic briefs, boxer briefs, or boxer shorts. T-shirts, sleeveless shirts (also known as singlets, tank tops, A-shirts, or vests), bikini underwear, thongs, G-strings, and T-fronts are items worn by both sexes.