A gastric ulcer is a sore or lesion that develops on the lining of the stomach. It is a type of peptic ulcer, a broad term that refers to any ulcer in the gastrointestinal (GI) tract caused by the erosion of the mucosal lining. Gastric ulcers specifically occur in the stomach, while duodenal ulcers occur in the first part of the small intestine (duodenum).