A melanocytic nevus (also known as nevocytic nevus, nevus-cell nevus, and commonly as a mole) is a usually noncancerous condition of pigment-producing skin cells. It is a type of melanocytic tumor that contains nevus cells. 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 body's pigmenting agent, melanin, is responsible for their dark color. Moles are a member of the family of skin lesions known as nevi (singular "nevus"), occurring commonly in humans. 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 2 decades of a person's life, with about 1 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.

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 (the latter 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. Common mole hair removal procedures include plucking, cosmetic waxing, electrolysis, threading, and cauterization.

Moles tend to appear during early childhood and during the first 30 years of life. They may change slowly, becoming raised, changing color, or gradually fading. Most people have between 30 and 40 moles, but some have as many as 600.

The number of moles a person has was found to have a correlation with telomere length. However, the relation between telomeres and aging remains uncertain.

The American Academy of Dermatology says that the vast majority of moles are benign. Data on the chances of transformation from melanocytic nevus to melanoma are controversial, but it appears that about 10% of malignant melanomas have a precursor lesion, of which about 10% are melanocytic nevi. Therefore, it appears that malignant melanoma quite seldom (1% of cases) has a melanocytic nevus as a precursor.

The cause of this condition is not clearly understood, but it is thought to result from a defect in embryologic development during the first 12 weeks of pregnancy. The defect is thought to cause a proliferation of melanocytes, the cells responsible for normal skin color. When melanocytes are produced at an extremely rapid rate, they form in clusters instead of spreading out evenly, resulting in abnormal skin pigmentation in some areas of the body.

Genes can influence a person's moles. Dysplastic nevus syndrome is a largely hereditary condition that 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 type of skin cancer. Dysplastic nevi are more likely than ordinary moles to become cancerous. While dysplastic nevi are common, and many people have a few of these abnormal moles, having more than 50 ordinary moles also increases the risk of developing melanoma.