High Quality Content by WIKIPEDIA articles! A diaper (in American and Canadian English) or a nappy (in Commonwealth English) is a kind of underwear that allows one to defecate or urinate in a discreet manner. When diapers become soiled, they require changing; this process is often performed by a second person such as a parent or caregiver. Failure to change a diaper on a regular enough basis can result in diaper rash.