A diaper /ˈdaɪpə(r)/ or a nappy is a type of underwear that allows the wearer to urinate or defecate without the use of 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.

Blue diaper syndrome is a rare, autosomal recessive metabolic disorder characterized in infants by bluish urine-stained diapers. It is also known as Drummond's syndrome, and hypercalcemia.
A cloth diaper or a cloth nappy is a reusable diaper made from natural fibers, man-made materials, or a combination of both. They are often made from industrial cotton which may be bleached white or left the fiber’s natural color. Other natural fiber cloth materials include wool, bamboo, and unbleached hemp. Man-made materials such as an internal absorbent layer of microfiber toweling or an external waterproof layer of polyurethane laminate (PUL) may be used. Polyester fabrics microfleece or suedecloth are often used inside cloth diapers as a "stay-dry" wicking liner because of the non-absorbent properties of those synthetic fibers.

A diaper bag or nappy bag is a storage bag with many pocket-like spaces that is big enough to carry everything needed by someone taking care of a baby while taking a typical short outing.

Diaper Genie, a creation of entrepreneur John Hall, is a baby diaper disposal system. The unit consists of a large plastic container with a plastic lid. The system seals diapers individually in a scented film to protect against germs and odors. By opening the lid on the top of the canister, a soiled diaper may be inserted into the "mouth" of the container. After inserting the diaper, the lid is replaced and twisted three full rotations to seal the diaper inside. After the container is filled with dirty diapers, it can be emptied by unlatching the bottom of the canister, where the diapers fall out still individually sealed. The resulting string of sealed diapers is colloquially known as a "diaper sausage".

Diapers.com was an online specialty retailer for baby products from 2005 to 2017. It was founded by Marc Lore and Vinit Bharara in Montclair, New Jersey. Initially named 1800DIAPERS, the company set out delivering consumables, such as diapers, wipes, and formula. It used Kiva robots for warehouse automation and a combination of UPS Ground and FedEx delivery. In late 2008, Diapers.com expanded its selection of baby products, including clothes, car seats, strollers, and toys.

Irritant diaper dermatitis is a generic term applied to skin rashes in the diaper area that are caused by various skin disorders and/or irritants.

Victor Mills was an American chemical engineer for the Procter & Gamble company. He is most credited for the creation of modern disposable diapers and the Pampers brand, production improvements for Ivory soap and Duncan Hines cake mix, and the production concept for Pringles. Within P&G he is regarded as the most productive technologist in the company's history. Therefore, when the company formed an honorary society for their engineers, it was named the Victor Mills Society.

A swim diaper or swim nappy is a diaper that is made for those who are incontinent, usually babies or toddlers, which is worn underneath a bathing suit, or as a bathing suit. Swim diapers can be reusable and disposable. They are not intended to be absorbent, but only to contain solid waste (feces); the lack of absorbency prevents the swim diaper from swelling with water.