A rash was any change to an individual's skin which affected its appearance or texture. A rash could often be the symptom of an illness.
In 2374, while on board the USS Prometheus, The Doctor claimed that one possible symptom of the Terothka virus was a rash. He said that some persons found the rash worse than the stomach cramps that could also result from the virus. (VOY: "Message in a Bottle")