rash, nonspecific term for an eruption of the skin. It may result from skin allergy, skin irritation, or skin disease, or it may be a symptom of a systemic disease like measles, smallpox, or scarlet fever. A rash may appear as discolored spots or a general redness, or as blisters or pustules, either flat or raised above the surface of the skin. The rash may cause skin irritation—itching, tingling, burning, or pain—or it may not cause any discomfort. The appearance and distribution of a rash are often important factors for consideration in diagnosing a particular disease.