Definition of comedoplural comedones play \ˌkä-mə-ˈdō-(ˌ)nēz\
:a small bump or blemish on the skin (as of the face or back) usually containing a plug of sebum in a skin pore: such as
or open comedo or closed comedo
but in keeping it simple---an open zit which has been exposed to the air is a blackhead. A closed zit is a white head. Pictures are worth a million words. See the picture below. And please listen to the video.
Here's to Good Health and Much Happiness,
Your Friend, Dr. Younger You :)