From Wikipedia, the free encyclopedia. An adult diaper (or adult nappy) is a diaper made to be worn by a person with a body larger than that of an infant or toddler. Diapers can be necessary for adults with various conditions, such as incontinence, mobility impairment, severe diarrhea or dementia. Adult diapers are made in various forms, including those resembling traditional child diapers, underpants, and pads resembling sanitary napkins (known as incontinence pads).