Making something good become cheap, coarse, or low in quality — and the state of having been made that way. People use it about art, language, religion, a tradition, a famous story: 'the vulgarization of a great novel by a bad film'. It carries a clear judgement, and often a snobbish one — the speaker is looking down at what has happened, and sometimes at the ordinary people they blame for it. Because of that, be careful using it about someone else's taste. There is a second, older and kinder use worth knowing: 'vulgarization' can mean putting difficult knowledge into plain language so ordinary people can understand it, with no insult meant. That use is common in French ('vulgarisation' = popular science writing) and shows up in older English writing, but most English readers today will hear only the negative sense, so say 'popularization' if that is what you mean. It is a formal, written word — you will meet it in criticism and essays, not in talk. British spelling: 'vulgarisation'.