There is no difference in meaning between flammable and inflammable. Both mean capable of burning or easy to ignite. But in all modern varieties of English, flammable is preferred.
Inflammable, derived from the verb inflame, is the original word. But because the first syllable is easily misinterpreted to be the common negative prefix in- (as in, for example, inescapable, invulnerable, inorganic), the word has always caused confusion. And because this word confusion can have dangerous real-world consequences, the shift from inflammable to flammable is welcome.
The less confusing flammable did not enter common use until the early 20th century, but it quickly became the prevalent spelling (see the Ngram at the bottom of this post). Inflammable is still common on product labels and appears from time to time in edited publications, but it fell out of favor around 1970.
Examples
These major publications use flammable:
He said the pattern indicated that a flammable liquid had been poured down the second-floor hallway . . . [New York Times]
But the wires in those days were flammable . . . [Guardian]
It’s highly flammable and burns much hotter than wood or other fuels . . . [Newsday]
They found a stove top had accidentally been left on in the kitchen, with flammable household items on top. [Sydney Morning Herald]
Instances inflammable in edited publications are difficult to find. But its derivatives inflame and inflammatory are common.
Ngram
This Ngram charts the use of flammable and inflammable in English-language books published from 1800 to 2000.
