1. A lesion of the skin or a mucous membrane; such as, the one lining the stomach or duodenum which is accompanied by formation of pus and necrosis of surrounding tissue, usually resulting from inflammation or ischemia (inadequate blood supply [circulation] to a local area because of a blockage of the blood vessels to the area).
2. An area of tissue erosion, for example, of the skin or lining of the gastrointestinal (GI) tract.

Due to the erosion, an ulcer is concave. It is always depressed below the level of the surrounding tissue.

Ulcers have a variety of causes and on the skin are often a result of irritation, as with bedsores, and they may become infected and inflamed as they grow.

3. A circumscribed inflammatory and often suppurating lesion on the skin or an internal mucous surface resulting in the necrosis of tissue.
4. A corrupting condition or influence.

An ulcer of the skin.

Ulcer resulting from injury to the urinary bladder by use of the cystoscope.

A cutaneous ulcer associated with diabetes mellitus.

Ulcer of the stomach or duodenum; a peptic ulcer.

Ulcer of the oral mucosa or of the skin, usually around the mouth or external nares; resulting from herpes simplex or, less often, herpes zoster.

A chronic ulcer that fails to heal.

An ulcer of the skin which is contaminated by feces.

An ulcer on the floor of the mouth, the ventral surface of the tongue, or the frenum (restraining structure) of the tongue.

Ulcer in the bladder that appears to heal only on the surface.

An ulcer of the oral mucosa caused by repeated trauma from a dental appliance or a sharp tooth.

An ulcer that forms as a result of varicose veins.

When thrombophlebitis (inflammation of a vein that occurs when a blood clot forms) develops in varicose veins, this leads to venous stasis and eventually edema (swelling of soft tissues as a result of excess water accumulation) and ulcer formation.

Venous stasis refers to the congestion and slowing of circulation in veins due to blockage by either an obstruction or high pressure in the venous system, usually best seen in the feet and legs.

Edema is often more prominent in the lower legs and feet toward the end of the day as a result of the pooling of fluid from the upright position maintained during the day. Upon awakening from sleeping, patients can have swelling around the eyes referred to as "periorbital edema".