When used as a medical term, keloids are excessive scars. They grow far past normal scarring for a wound and are exceedingly difficult to treat - often surgery to remove keloids will only worsen the problem. Most sufferers of keloiding are dark-skinned, as there's a relation between melanin and the amount of scarring. Keloiding is caused by trauma to the skin, and more often form on body parts where constant use irritates the wound.

Keloid is also used as a term in body modification, where it refers simply to raised scars, often a goal of scarification, and the act of raising them through irritation of the scab. Also, excessive scar tissue caused by body piercing will often be called keloids. While it's certainly possible to scar too much due to bad piercing techniques (see piercing gun), they're not keloids in the medical sense.