Red Rash Pictures and Symptoms A rash is a condition that appears on the skin, usually as red or pink papules or patches. Rashes can be caused by viruses, bacteria, allergies, or other illnesses. This article will detail the definition, causes, symptoms, and treatment of erythema from multiple perspectives. Definition of rash: A rash is a skin condition that usually appears as one or more red or pink patches on the surface of the skin. The red rash may be accompanied by symptoms such as itching, burning, or pain. This skin symptom can be a symptom of a variety of diseases