Nevus is a medical term for birthmark or mole. A nevus is the most common skin abnormality, generally appearing as a darken spot anywhere on the body. Nevi serve no apparent function and are generally harmless.

Nevomelanocytes are pigment producing cells usually evenly distributed in human skin. In the area of a nevus, these cells are found in large concentration or grouped together. This creates the dark color or spot on the skin. Nevi range in color from a light tan to almost black depending on the type and pigment concentration.

The skin of a nevus can be different than the rest of the body’s skin. In addition to more pigment, some types of nevi produce more hair. The formation of a nevus can disturb the production of sweat glands hindering the affected area’s ability to sweat. They often lack subcutaneous fat which makes the skin thinner and less durable. Sometimes a nevus produces thicker or rough pebble textured skin.

A nevus present at birth is congenital, often called a birthmark. One that develops after birth is called an acquired nevus, or a mole. Birthmarks develop in the womb within the first 12 weeks of gestation. There is no evidence to suggest that they are hereditary and they vary greatly in size and location.

Nevi are generally harmless. Usually a benign (showing no signs of cancer) growth, they present a visual abnormality but no inherent medical risk. An atypical nevus might share some of the features of melanoma without showing any signs of cancer. Any skin abnormality should be checked by a doctor to ensure it is benign and not malignant (cancerous).

Large conspicuous nevi are a problem for some as their appearance can generate unwanted curiosity or attention. Although about 1 in 100 babies are born with a nevus, almost everyone will have 20 to 40 small moles on their body by adulthood. Because a nevus can occur on the face or other conspicuous body parts, they are sometimes bothersome or embarrassing to those with them. Support groups exist for persons uncomfortable with their physical appearance due to nevi. Some nevi can be removed by a plastic surgeon but the process can be uncomfortable and leave large visible scars

Classifications of acquired nevi include a junctional nevus. This type of mole is flat, brown to black in color and slightly raised. A compound nevus is similar to the junctional with the exception being that coarse hairs may be present.

A blue nevus is dome shaped and blue to black in color. Blue nevus usually present in childhood or adolescence. The spitz nevus is brown to pink in color and dome shaped. They are most commonly located on head, neck or arms and present in childhood. The spitz nevus can appear similar to melanoma causing initial concern but are by definition, benign.