A melanocytic nevus (also known as nevocytic nevus, nevus-cell nevus and commonly as a mole) is a type of melanocytic tumor that contains nevus cells. Some sources equate the term mole with "melanocytic nevus", but there are also sources that equate the term mole with any nevus form.The majority of moles appear during the first two decades of a persons life, with about one in every 100 babies being born with moles. Acquired moles are a form of benign neoplasm, while congenital moles, or congenital nevi, are considered a minor malformation or hamartoma and may be at a higher risk for melanoma. A mole can be either subdermal (under the skin) or a pigmented growth on the skin, formed mostly of a type of cell known as a melanocyte. The high concentration of the bodys pigmenting agent, melanin, is responsible for their dark color. Moles are a member of the family of skin lesions known as nevi and can occur in all mammalian species, but have been documented most extensively in humans, dogs, and horses. Signs and symptoms
According to the American Academy of Dermatology, the most common types of moles are skin tags, raised moles and flat moles. Benign moles are usually brown, tan, pink or black (especially on dark-colored skin). They are circular or oval and are usually small (commonly between 1–3 mm), though some can be larger than the size of a typical pencil eraser (>5 mm). Some moles produce dark, coarse hair.
Dysplastic nevus syndrome is a largely hereditary condition which causes a person to have a large quantity of moles (often 100 or more) with some larger than normal or atypical. This often leads to a higher risk of melanoma, a serious skin cancer. Dysplastic nevi are more likely than ordinary moles to become cancerous. Dysplastic nevi are common, and many people have a few of these abnormal moles. Having more than 50 ordinary moles increases the risk of developing melanoma.In the overall population, a slight majority of melanomas do not form in an existing mole, but rather create a new growth on the skin. Somewhat surprisingly, this also applies to those with dysplastic nevi. They are at a higher risk of melanoma occurring not only where there is an existing mole, but also where there are none. Such persons need to be checked regularly for any changes in their moles and to note any new ones. Sunlight
Ultraviolet light from the sun causes premature aging of the skin and skin damage that can lead to melanoma. Some scientists hypothesize that overexposure to UV, including excessive sunlight, may play a role in the formation of acquired moles. However, more research is needed to determine the complex interaction between genetic makeup and overall exposure to ultraviolet light. Some strong indications that this is so (but falling short of proof), are:

The relative lack of moles on the buttocks of people with dysplastic nevi.