1) A lesion on the surface of the skin or a mucous surface, produced by the sloughing of inflammatory necrotic tissue.

2) A break in skin or mucous membrane with loss of surface tissue, disintegration and necrosis of epithelial tissue, and often pus.

3) A break on the skin, in the lining of an organ, or on the surface of a tissue. An ulcer forms when the surface cells become inflamed, die, and are shed. Ulcers may be linked to cancer and other diseases.