Flammable

A substances in the form of a GAS, Vapors, Liquid, solid or a mixture that can react exothermically (producing heat energy) with air (oxygen) when ignited.

Flammability is defined as how easily something will burn or ignite, causing FIRE or COMBUSTION

Flammable Liquid Warning Sign

Classification of flammability

The ratings are as follows: (Source NFPA & Hazardous Materials Identification System)

Rating	Degree of flammability	Examples
0	Materials that will not burn	WATER
1	Materials that must be preheated before they will ignite	LUBRICATING OIL, COOKING OILS
2	Materials that must be moderately heated or exposed to relatively high ambient temperatures before they will ignite	DIESEL FUEL
3	Liquids and solids that can ignite under almost all temperature conditions	GASOLINE, ACETONE
4	Materials which will rapidly vaporize at atmospheric pressure and normal temperatures, or are readily dispersed in air and which burn readily	NATURAL GAS, PROPANE,BUTANE