A lesion on the surface of the skin or a mucous membrane characterized by a superficial loss of tissue. Ulcers are most common on the skin of the lower extremities and in the gastrointestinal tract, although they may be encountered at almost any site. The diverse causes of ulcers range from circulatory disturbances or bacterial infections to complex, multifactorial disorders. The superficial tissue sloughs, leaving a crater that extends into the underlying soft tissue, which then becomes inflamed and is subject to further injury by the original offender or secondary infection.