The ease with which a material (gas, liquid, or solid) will ignite either spontaneously (pyrophoric) or from exposure to a high-temperature environment (autoignition) or a spark or open flame. It also involves the rate of spreading of a flame once it has started. The more readily ignition occurs, the more flammable the material; less easily ignited materials are said to be combustible, but the line of demarcation is often indefinite and depends on the state of subdivision of the material, as well as its chemical nature