Diarrhea: A common condition that involves unusually frequent and liquid bowel movements. The opposite of constipation. There are many infectious and noninfectious causes of diarrhea. Persistent diarrhea is both uncomfortable and dangerous to the health because it can indicate an underlying infection and may mean that the body is not able to absorb some nutrients due to a problem in the bowels. Treatment includes drinking plenty of fluids to prevent dehydration and taking over-the-counter remedies. People with diarrhea that persists for more than a couple days, particularly small children or elderly people, should seek medical attention.