Cancer, melanoma: A skin cancer that begins in cells called melanocytes that can grow together to form benign (not cancerous) moles. A change in size, shape, or color of a mole can be a sign of melanoma. It can be cured if detected early, before spread (metastasis) to other areas. Diagnosis is confirmed by a biopsy of the abnormal skin. Sun exposure can cause skin damage that can lead to melanoma.