malignant neoplasm derived from cells that are capable of forming melanin, which may occur in the skin of any part of the body, in the eye, or, rarely, in the mucous membranes of the genitalia, anus, oral cavity, or other sites; occurring mostly in adults and may originate de novo or from a pigmented nevus or malignant lentigo; frequently metastasize widely, and the regional lymph nodes, liver, lungs, and brain are likely to be involved.