Concave sore on the skin or lining of an organ, with well-defined, sometimes raised edges. Erosion of surface tissue may extend to deeper layers. The main symptom is pain. The term most often refers to peptic ulcer but also includes skin ulcer, common on legs with varicose veins and the feet of people with diabetes mellitus (when nerve damage has reduced sensation), and decubitus ulcer (bedsore or pressure sore). Other causes include infection, trauma (e.g., burn, frostbite), improper nutrition (e.g., thiamine deficiency), and cancer (likely in ulcers hard to the touch). Skin ulcers over a month old should be checked for cancer, especially after middle age.