Hamartoma of the skin (synonymous with "nevus") is a visible, sharply circumscribed, often congenital, but also postnatally manifesting, long-term malformation of the skin or mucosa, which is characterized by excess, more rarely also by underdevelopment or malformation, of one or more components of skin or mucosa, and which etiopathogenetically reflects a cutaneous mosaic (see also under malformation).
Hamartomas are named after the predominant structure, e.g., sebaceous gland s. Nevus sebaceus, sweat glands s. Hamartoma ekkrines, melanocytes s. Nevus melanocytärer, connective tissue s. Connective tissue naevus, hamartoma fibrinous, blood vessels s. Hamangioma, nevus flammeus ,s.a. malformation vascular, u.s.w. compare also malformation. Combined malformations are possible, whereby the predominant tissue type gives the name. Systematized hamartomas are those which are distributed over a body region. They follow in their arrangement either a dermatome, but more frequently the Blaschko lines (classification see under nevus).