A keloid is a scar consisting mainly of type I and some type III collagen, along with elastin, fibronectin, and proteoglycans. The scar presents as a raised area at the site of a former wound. Keloids are benign tumors that can can expand well beyond the area of the original wound. They are firm and rubbery or shiny in appearance, and range from pink to red to dark brown in color. Keloid scars are often accompanied by sharp pain and itching, and in some cases, they can impair movement, especially if located over a joint.

A keloid can form from any type of skin injury or abrasion, including cosmetic piercings, surgery, insect bites, burns, acne, and shaving irritation. More rarely, one can form spontaneously. Keloids are fifteen times more likely in people with very high skin pigmentation, and those of African descent may be have an increased risk of developing keloids, regardless of their skin color. People can develop a keloid at any age, though they are less likely in children under 11.

There is no surefire cure for keloid scars. In extreme cases, such as when a keloid becomes infected and turns into an ulcer, surgery may be performed, but there is at least a 50% chance of recurrence in surgically removed keloids, and the recurring scar may be even larger than the original one. The earlier one begins treatment to reduce or eliminate keloids, the better the prognosis. When possible, prevention is the best course of action against keloids. A person with a history of keloids should avoid cosmetic piercings or other body modification, including non-essential surgeries, and should take special care with healing wounds.

Keloids may be treated with a variety of topical applications, including aspirin paste, tea tree oil, silicone gel dressings, and natural mucin. Compression therapy, in which bandages are tightly worn over the scar, can reduce the keloid's appearance. More invasive treatment options include cryosurgery, laser or radiation therapy, and injections of steroids, interferon, or the chemotherapy drug fluorouracil.