A carbuncle is an abscess larger than a boil, usually with one or more openings draining pus onto the skin. It is usually caused by bacterial infection, most commonly Staphylococcus aureus. The infection is contagious and may spread to other areas of the body or other people.

A carbuncle is made up of several skin boils. The infected mass is filled with fluid, pus, and dead tissue (Tissue (biology)). Fluid may drain out of the carbuncle, but sometimes the mass is so deep that it cannot drain on its own. Carbuncles may develop anywhere, but they are most common on the back and the nape of the neck. Men get carbuncles more often than women. Because the condition is contagious, family members may develop carbuncles at the same time. Often, the direct cause of a carbuncle cannot be determined. Things that make carbuncle infections more likely include friction from clothing or shaving, generally poor hygiene and weakening of immunity (Immunity (medical)). For example, persons with diabetes and immune system diseases are more likely to develop staphylococcal infections.

The carbuncle may be the size of a pea or as large as a golf ball. It may be red and irritated, and might hurt when touched. It may also grow very fast and have a white or yellow center. It may crust or spread to other skin areas. Sometimes, other symptoms may occur, such as fatigue (fatigue (medical)), fever and a general discomfort or sick feeling. Itching may occur before the carbuncle develops.

The word is believed to have originated from the Latin: carbunculus, originally a small coal; diminutive of carbon-, carbo: charcoal or ember, but also a carbuncle stone (carbuncle (gemstone)), "precious stones of a red or fiery colour", usually garnets.