Irritant diaper dermatitis
Irritant diaper dermatitis (IDD, also called a diaper rash or nappy rash) is skin rash in the diaper (in British and Australian English "nappy") area, caused by various skin disorders and/or irritants. It is the most common skin disorder seen in infants.
Source: Wikipedia — Irritant diaper dermatitis (CC BY-SA 4.0)