Flammable and inflammable both mean capable of catching fire easily, despite the in- prefix making inflammable look like it should mean the opposite. Flammable was deliberately created as a clearer word for warning labels, so nobody would misread inflammable as fireproof.
The core difference
Both words describe something that burns easily; flammable was coined to avoid the dangerous ambiguity of inflammable.
- flammable — capable of catching fire easily: Gasoline is highly flammable, so store it away from any heat source.
- inflammable — easily set on fire (the older, formal term): The warehouse stored inflammable liquids in a sealed room.
Why they mean the same thing
Inflammable comes from the Latin inflammare, meaning to set on fire, and the in- here is intensive rather than negative, unlike the in- in words such as inedible. Flammable was later formed as a back-formation from inflammable, created specifically so warning labels could not be misread as meaning not flammable. Because of that risk, flammable is now the standard word on hazard labels and in everyday use, while inflammable survives mainly in older or more formal writing.
A memory trick: think of inflammable as inflame-able, since both come from the same root. If in doubt, use flammable, since it is the word least likely to be misunderstood.
