Blackhead: A familiar term for what is medically called an open comedo. A comedo, the primary sign of acne, consists of a dilated (widened) hair follicle filled with keratin squamae (skin debris), bacteria, and sebum (oil).

An open comedo has a wide opening to the skin and is capped with a blackened mass of skin debris. By contrast, a closed comedo has an obstructed opening to the skin and may rupture to cause a low-grade skin inflammatory reaction in the area. The common name for a closed comedo is a whitehead.

Acne occurs when sebaceous glands of the skin begin to secrete oil during puberty. These glands are stimulated by male hormones that are produced in the adrenal glands of both boys and girls. The oil lubricates and protects the skin. Under certain circumstances, cells that are close to the openings of the sebaceous glands block the openings. This causes a buildup of oil underneath the skin. Bacteria feed on this oil, multiply, and cause the surrounding tissues to become inflamed.

If the inflammation is near the skin's surface, a pustule is formed; if it is deeper, a papule (pimple); if it is deeper still it becomes a cyst. If the oil breaks though to the surface, the result is a whitehead. If the oil becomes oxidized (that is, acted on by oxygen in the air), the oil changes from white to black, and the result is a blackhead. This is how the two types of comedones develop.