Flammable and inflammable are synonyms, both meaning “susceptible of or capable of catching fire and burning rapidly.” Inflammable was the word of choice for a long time, but fire-fighting associations and insurers, apparently concerned that the in- prefix would be misunderstood to mean “un-” or “non-” (which in another in prefix it does) decided to remove all doubt by labeling materials, gasoline trucks, and other things that can burn flammable. Both words are still in use, and both are Standard. Nonflammable, incombustible, and noncombustible are antonyms of flammable and inflammable: they mean “fireproof.” But see FIREPROOF.
