What's the Difference Between Flammable and Inflammable?

These two words both pertain to fire and mean the same thing

Both flammable and inflammable mean a substance readily burns.

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Flammable and inflammable are two words that cause confusion. You can tell both words pertain to flames, but it's difficult to know whether they mean the same thing or are opposites. The truth is, flammable and inflammable mean exactly the same thing: a substance burns easily or readily catches fire.

So, why are there two different words? According to Merriam-Webster's Dictionary of English Usage, back in the 1920s, the National Fire Protection Association urged people to start using the word "flammable" rather than "inflammable" (which was the original word) because they were concerned some people might think inflammable meant not-flammable or nonflammable.

Actually, the in- in inflammable was derived from the Latin preposition en-, which serves as an intensifier (as in enflamed and engulfed), not the Latin prefix meaning un-, meaning "not." It's not like everyone knew the derivation of the word, so the change probably made sense. However, confusion persists today regarding which word to use.

While flammable is the preferred modern term for a material that catches fire readily, inflammable has the same meaning. The opposite, a material that won't burn easily, is either not-flammable or non-flammable.

Examples of flammable materials include wood, kerosene, and alcohol. Examples of nonflammable materials include helium, glass, and steel. While it may surprise you, another example of a non-flammable substance is oxygen—which, as an oxidizer, is instead combustible.