Flammable means catching fire easily. Highly flammable, flammable liquids, a flammable material.
Inflammable means exactly the same thing — the in- is intensifying rather than negative — and because that was lethally confusing, safety bodies deliberately promoted flammable in the twentieth century and it has largely won. Non-flammable is the opposite. This is one of the very few cases where a word was changed on purpose to stop people dying. Note flammability; and inflame, from the same Latin root.
1adjectiveflammableCatching fire easily.
“Keep flammable liquids like petrol away from the barbecue.”
synonyms: inflammable, combustible
flammable is a quality of catching fire.
a back-formation from inflammable, which is used to avoid confusion with non-flammable, as the prefix in- is often used to mean "un-; non-", although it was originally meant in a sense closely related to en-.
Etymology from Wiktionary, CC BY-SA