Melanocytic nevus

Somewhere on the skin, cells that normally keep to themselves have crowded into one place, and that spot is what everyday speech calls a mole. Its medical name is melanocytic nevus, and the cells doing the crowding are melanocytes, which live in the stratum basale, one of the layers making up the epidermis, the sheet that forms the outside of the skin.

What a melanocyte does for a living is make melanin, and the shade that skin and hair end up with depends on that pigment, which also helps shield the surrounding living cells from ultraviolet light. Think of these cells as seeds sprinkled over a sheet of paper: across almost all the skin they sit apart, one here, one there. Where several end up piled on the same patch of paper, the spot becomes noticeable, and a noticeable spot of that sort is what gets called a mole. Nearly every one of those piles is harmless, benign in medical language, and only in a small number of cases does one of these ordinary moles end up turning into melanoma, the most serious of the skin cancers.