Medical Encyclopedia by MedlinePlus, National Library of Medicine, U.S. National Institutes of Health. Provides basic information on melanoma skin cancer, its medical definition, causes, risk factors, symptoms, diagnosis, treatment, prognosis, possible complications, prevention, and when to contact a medical professional. Melanoma is the most dangerous type of skin cancer. It is the leading cause of death from skin disease. Four different types of melanoma are discussed. Melanoma spreads rapidly, and early diagnosis is crucial. The risk of developing melanoma increases with age. The principal cause of melanoma is exposure to sunlight.