An ulcer is a crater-like sore on the skin or mucous membrane. Ulcers form when the top layers of skin or tissue have been removed. They can occur in the mouth, stomach, and other parts of the body.
Ulcers can be caused by inflammation or infection. Some ulcers may be caused by a cancer.
Habif TM. Principles of diagnosis and anatomy. In: Habif TP, ed. Clinical Dermatology. 5th ed. Philadelphia, Pa: Mosby Elsevier; 2009:chap 1.