The autoignition temperature of a substance is the lowest temperature at which it will spontaneously combust in a normal atmosphere, without an external source of ignition, such as a flame or spark. The lowest temperature at which a substance will ignite in the presence of an external source of ignition is known as its flash point.

Autoignition temperatures are measured using the same closed cup apparatus used for measuring flash points.

The autoignition temperature of paper, 451 degrees Fahrenheit, is well known because of the popular novel Fahrenheit 451 by author Ray Bradbury.