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 almost 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.
There is a slight distinction between inflammable and flammable materials, though, that is important to note for safety. A material that is flammable can be set on fire, while a material that is inflammable is capable of bursting into flames without an external source of ignition.
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.