Comedogenic” means that something is likely to clog skin pores, which can then lead to acne lesions.
“Non-comedogenic” means that something is unlikely to clog pores.
The root word of each term is “comedone,” which is the medical term for a clogged pore. Natural processes in our skin can lead to clogged pores, but ingredients in cosmetics can sometimes be the culprit as well. Comedogenic ingredients are ingredients that lead to clogged pores, called "comedones." It is thought that comedogenic ingredients stimulate the overproduction of skin cells inside a skin pore, and this leads to a pore blockage, and ensuing acne.
The tool that product formulators have is to simply avoid using ingredients that have been shown to be comedogenic in laboratory testing. The first list of results for the comedogenic potential of ingredients was published in a 1989 study in the Journal of the Society of Cosmetic Chemists:
The best way to determine if a product might break you out is to look at the ingredient list and avoid using products that contain highly comedogenic ingredients within the first seven (7) ingredients listed on the label.