The spontaneous ignition of a substance, like fuel in an engine, without an external spark or flame being applied to it.
From Greek 'auto' (self) and Latin 'ignitio' (setting on fire). The term emerged in the early 20th century with advances in internal combustion engines, describing the unwanted self-ignition of fuel.
Autoignition is why high-octane gasoline exists—it resists spontaneously exploding in your engine before the spark plug fires. Diesel engines actually rely on autoignition as their primary ignition method, making them fundamentally different from gasoline engines.
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