A diaper (, North American English) or a nappy (British English, Australian English, and Hiberno-English) is a type of underwear that allows the wearer to urinate or defecate without using a toilet, by absorbing or containing waste products to prevent soiling of outer clothing or the external environment. When diapers become wet or soiled, they require changing, generally by a second person such as a parent or caregiver. Failure to change a diaper on a sufficiently regular basis can result in skin problems around the area covered by the diaper.

Diapers are made of cloth or synthetic disposable materials. Cloth diapers are composed of layers of fabric such as cotton, hemp, bamboo, microfiber, or even plastic fibers such as PLA or PU, and can be washed and reused multiple times. Disposable diapers contain absorbent chemicals and are thrown away after use.

Diapers are primarily worn by infants, toddlers who are not yet toilet trained, and by children who experience bedwetting. They are also used by adults under certain circumstances or with various conditions, such as incontinence. Adult users can include those of advanced age, patients bed-bound in a hospital, individuals with certain types of physical or mental disability, and people working in extreme conditions, such as astronauts. It is not uncommon for people to wear diapers under dry suits.