A carcinogen is a substance that can cause cancer. It might be a chemical, a kind of radiation, or another thing that, with enough contact over time, raises the chance that cells go wrong. Cigarette smoke contains carcinogens. Some workplace chemicals and some kinds of pollution are labelled this way. The word is careful, and that care matters: ‘can cause’ is not the same as ‘will give everyone cancer after one touch’. Scientists test, argue, and rank how strong the evidence is; a thing can be a known carcinogen, a probable one, or still under study. You will hear this on health warnings, in the news, and in science class. It is a formal, serious word. People do not use it for a food they merely dislike, and it is not the name of the illness itself — the illness is cancer; the carcinogen is something that can help start it.