A diaper (in North America) or nappy (in the United Kingdom, many Commonwealth countries and Ireland) is an absorbent garment worn by individuals who are incapable of controlling their bladder or bowel movements, or are unable or unwilling to use a toilet. The purpose of a diaper is to absorb moisture and contain mess so that the wearer can remain dry and comfortable after wetting or soiling themselves. When diapers become full and can no longer hold any more waste, they require changing; this process is often performed by a secondary person such as a parent or caregiver. Failure to change a diaper on a regular enough basis can result in diaper rash.

Diapers have been worn throughout human history, and can be made of either cloth or disposable materials. Whereas cloth diapers are comprised of layers of fabric such as terry towelling and can be washed and reused multiple times, disposable diapers contain absorbent chemicals and are thrown away after use. The decision to use cloth or disposable diapers is a controversial one, due to issues ranging from convenience, health, cost, and their effect on the environment. Currently, disposable diapers are the most commonly used, with Pampers and Huggies being the most well-known brands in the industry. Plastic pants can be worn over diapers to avoid leaks. Diapers are primarily worn by children who are not yet potty trained or suffer from bedwetting. However, they can also be used by adults who suffer from incontinence or in certain circumstances where access to a toilet is unavailable. These can include the elderly, those with a physical or mental disability, and people working in extreme conditions such as astronauts. Diapers are usually worn out of necessity rather than choice, although there are exceptions; people such as infantilists and diaper fetishists wear diapers recreationally for comfort, emotional fulfillment, or sexual gratification.