Hemorrhoids: Dilated (enlarged) veins in the walls of the anus and sometimes around the rectum, usually caused by untreated constipation but occasionally associated with chronic diarrhea. Symptoms start with bleeding after defecation. If untreated, hemorrhoids can worsen, protruding from the anus. Treatment involves changing the diet to prevent constipation and avoid further irritation, the use of topical medication, and sometimes surgery. Also known as piles.