Wikipedia: A comedo is a clogged hair follicle (pore) in the skin. Keratin (skin debris) combines with oil to block the follicle. A comedo can be open (blackhead) or closed by skin (whitehead), and occur with or without acne. The word comedo comes from Latin to suggest the worm-like look of a blackhead that has been secreted. The plural of comedo is comedones.

The chronic inflammatory condition that usually includes both comedones and inflamed papules and pustules (pimples or zits) is called acne. Infection causes inflammation and the development of pus. Whether or not a skin condition classifies as acne depends on the amount of comedones and infection.

Comedo-type ductal carcinoma in situ (DCIS) is not related to the skin conditions discussed here. DCIS is a non-invasive form of breast cancer, but comedo-type DCIS may be more aggressive and so may be more likely to become invasive.