Merkel cell carcinoma: An infrequent but highly malignant type of skin cancer. Characteristically starts in a sun-exposed area (of the head, neck, arms or legs) in whites 60-80 years of age as a firm, painless, shiny lump that can be red, pink, or blue in color and vary in size from less than a quarter of an inch (a half cm) to more than two inches (5 cm) in diameter. The tumor grows rapidly and often metastasizes (spreads) to other parts of the body. Even relatively small tumors are capable of metastasis, particularly to the regional (nearby) lymph nodes. Merkel cell carcinoma follows an aggressive course like that of melanoma, and has a predilection to spread to (in order of frequency) liver, bone, brain, lung, and skin. The prognosis (outlook) is accordingly poor.