An ulcer (from Latin ulcus) is an open sore of the skin, eyes or mucous membrane, often caused by an initial abrasion and generally maintained by an inflammation and/or an infection.

Peptic ulcer is a non-malignant ulcer of the stomach (called gastric ulcer) or duodenum (called duodenal ulcer). By far most instances are now known to be due to Helicobacter pylori, a spiral-shaped bacterium that lives in the acid environment of the stomach. These ulcers can also be caused or worsened by drugs such as Aspirin and other NSAIDs.