Keloid, also known as keloid disorder and keloidal scar, is the formation of a type of scar which, depending on its maturity, is composed mainly of either type III (early) or type I (late) collagen. It is a result of an overgrowth of granulation tissue (collagen type 3) at the site of a healed skin injury which is then slowly replaced by collagen type 1. Keloids are firm, rubbery lesions or shiny, fibrous nodules, and can vary from pink to the color of the person's skin or red to dark brown in color. A keloid scar is benign and not contagious, but sometimes accompanied by severe itchiness, pain, and changes in texture. In severe cases, it can affect movement of skin. In the United States keloid scars are seen 15 times more frequently in people of sub-Saharan African descent than in people of European descent. There is a higher tendency to develop a keloid among those with a family history of keloids and people between the ages of 10 and 30 years.

Keloids should not be confused with hypertrophic scars, which are raised scars that do not grow beyond the boundaries of the original wound.