pyrophoric

adj. spontaneously igniting in air

A pyrophoric substance is a substance that ignites spontaneously: that is, its autoignition temperature is below room temperature. Examples are iron sulfide and many reactive metals including uranium, when powdered or sliced thinly. Pyrophoric materials are often water reactive as well and will ignite when they contact water or humid air. They can be handled safely in atmospheres of argon or (with few exceptions) nitrogen. Most pyrophoric fires should be extinguished with a class D fire extinguisher.
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(a.)
Alt. of Pyrophorous
  

a material that automatically ignites on exposure to oxygen or oxygen containing species.

Catches fire spontaneously when exposed to air. For example, powdered potassium metal is pyrophoric.