spontaneous combustion

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Ignition of a substance, such as oily rags or hay, caused by a localized heat-increasing reaction between the oxidant and the fuel and not involving addition of heat from an outside source.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Combustion without an external source of heat.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun ignition of a substance (as oily rags) resulting from an internal oxidation process