congenital skin lesion, or birthmark, caused by abnormal pigmentation or by proliferation of blood vessels and other dermal or epidermal structures. Nevi may be raised or may spread along the surface of the skin. In other types, such as the blue nevus, proliferative tissue is buried deep within the dermis, the lower layer of the skin.

Most types of nevi are formed by the overgrowth of normal skin components that retain their usual functions. Some nevi, however, are precancerous and lose their normal organization when they become malignant. Premalignant nevi include the sebaceous nevus, a congenital formation containing hair follicles and sebaceous glands, and the giant pigmented, or bathing trunk, nevus, a large, irregular, dark brown or black patch associated with malignant melanoma. Some pigmented nevi, such as the blue nevus and the junctional nevus, may be associated with skin cancers (cancer) but are not widely considered precancerous. Other pigmented nevi may be associated with systemic diseases; café-au-lait spots, light brown spots that can occur anywhere on the body, occur naturally in 10 percent of the white population but may also be a symptom of Albright's disease. Mongolian spot, a blue-black patch on the lower back, occurs in 90 percent of American Indian, black, and Asiatic persons.

Although pigmented nevi are among the most common of these skin growths, other varieties of nevus are also known. Vascular nevi, or hemangiomas, produce the familiar “port wine stain” birthmark; they are composed of blood vessels and often resolve themselves with time. Epidermal nevi, of which the sebaceous nevus is an example, develop from the proliferation of normal epidermal tissues and are usually flesh-coloured or yellowish.