Blackhead: A familiar term for what is medically called an open comedo. A comedo, the primary sign of acne, consists of a widened hair follicle filled with skin debris, bacteria, and oil called sebum. A blackhead has a wide opening to the skin and is capped with a blackened mass of skin debris. In contrast, a closed comedo, commonly called a whitehead, has an obstructed opening to the skin and may rupture to cause a low-grade skin inflammatory reaction in the area.
Kasper, D.L., et al., eds. Harrison's Principles of Internal Medicine, 19th Ed. United States: McGraw-Hill Education, 2015.