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Ulceration: a condition in which the epidermis that covers a portion of the primary melanoma is not intact. Ulceration is determined by microscopic evaluation of the tissue by a pathologist, not by what can be seen with the naked eye.
Ultraviolet light: See Ultraviolet radiation.
Ultraviolet (UV) radiation: Also called ultraviolet light. The part of sunlight that is invisible to the human eye. Some wavelengths of UV radiation filter through the earth's atmosphere and enter the body through the skin and eyes. UV radiation can burn the skin and cause melanoma and other types of skin cancer.
UVA rays: Long wavelength ultraviolet (UV) rays (320 to 400 nm) given off by the sun, tanning beds, and sunlamps. They enter the skin more deeply than UVB rays, cause premature aging of the skin, and are believed to cause skin cancer.
UVB rays: Medium wavelength (290 to 320 nm) "burning rays" of the sun that are the primary cause of sunburn. They are considered the main cause of basal and squamous cell carcinoma, as well as a significant cause of melanoma.
UVC rays: Short wavelength ultraviolet rays (200 to 290 nm) absorbed by the earth's ozone layer and so do not reach the earth.
UV index: A daily prediction of the strength of the sun's ultraviolet (UV) rays for a region.