Tattoo: The permanent insertion of ink below the skin, using a sharp instrument. Humans have done tattooing for cosmetic and ritual purposes since at least the Neolithic era. Today the practice is made relatively safe by the use of nonreactive pigments; sterile, disposable needles; and sterile work conditions. Without these refinements, inks may cause inflammation, and infection is an ever-present danger. Persons who are prone to keloids should be aware that tattoos can trigger these heaped-up disfiguring scars. Ink lines may also spread or change color over the years, a fact of special concern for those interested in so-called 'permanent cosmetics' (tattooed lip color, eyebrows, eyeliner, and the like).