Flammable, Inflammable, Non-flammable

'Inflammable' and 'flammable' mean the same thing. The 'in' prefix has crept in from the Latin source word.
However, there is a subtle difference in the usage:

• Flammable - Matter catches fire when it comes in contact
  
• Inflammable - Matter catches fire without a source of ignition; for example - by excessive pressure, sudden impact, reaction to the environment.

The opposite of 'flammable' is 'non-flammable', not 'non-inflammable'.
So much for our lingua franca.