Ulcer: A lesion that is eroding away the skin or mucous membrane. Ulcers can have various causes, depending on their location. Ulcers on the skin are usually due to irritation, as in the case of bedsores, and may become inflamed and/or infected as they grow. Ulcers in the gastrointestinal tract were once attributed to stress, but most are now believed to be due to infection with the bacterium Helicobacter pylori. GI ulcers, however, are often made worse by stress, smoking, and other noninfectious factors.