Something nonflammable does not catch fire easily, or does not burn at all. You meet this word on clothes, building materials, and bottles of chemicals. Safety labels like it because it is the clear opposite of flammable (able to burn). Though actually, “hard to burn” and “will not burn at all” are not quite the same: some things are hard to light, but they still burn once they start. You may also see non-flammable with a hyphen; that is the same word. The messy part is inflammable. It looks like “not flammable,” but it means can burn — the same as flammable. If you want “will not burn,” nonflammable is the word that does that job. It sounds a bit formal. You see it on labels and in science class more than in chat.