Nevus (or naevus, plural nevi or naevi, from nævus, Latin for "birthmark") is the medical term for sharply-circumscribed and chronic lesions of the skin. These lesions are commonly named birthmarks and moles. Nevi are benign by definition. However, 50% of malignant melanomas (a skin cancer) arise from pre-existing naevi. Using the term nevus and nevi loosely, most physicians and dermatologists are actually referring to a variant of nevus called the "Melanocytic nevus", which are composed of melanocytes. Histologically, melanocytic nevi are differentiated from lentigines (also a type of benign pigmented macule) by the presence of nests of melanocytes, which lentigines (plural form of lentigo) lack.