An aggravation is something that makes a situation or condition worse. Getting hot and sweaty might result in an aggravation of your poison ivy rash.
An aggravation of symptoms in a hospital patient is a bad sign — things are becoming more serious. Another, less formal way to use the noun aggravation is to mean something that annoys or bothers you. You might say, "Having to take the bus is such an aggravation — I'd much rather drive my car." The Latin root of aggravation is aggravare, "make heavier," or "increase in oppressiveness."