Diaper rash: An inflammatory reaction localized to the area of skin usually covered by the diaper. It can have many causes, including infections (yeast, bacterial, or viral), friction irritation, chemical allergies (perfumes, soaps), sweat, decomposed urine, and plugged sweat glands. Most diaper rash problems can be solved by cleansing the skin with nonperfumed, gentle products; changing diapers frequently; and exposing the affected skin area to air. Commercially available diaper rash ointments may be helpful for prevention but may actually cause further irritation if used on the inflamed areas. Also called diaper dermatitis.