Inflammable and flammable are two words that are frequently
misinterpreted. Some people mistake the words as having opposite
meanings. In reality, flammable and inflammable mean exactly the
same thing—capable of burning.
Inflammable precedes
the word flammable and has always meant 'tends to catch fire
easily'.
The US National Fire
Protection Association urged Americans during the 1920s to start
using the word flammable to avoid confusion, because they
thought people may mistake inflammable as meaning not being able
to burn. Flammable has since become the preferred alternative on
warning labels.
Incidentally, non-flammable
is the opposite, meaning not flammable.