1. Ignition
   A combustible mixture is triggered by an external ignition source, initiating chain reactions that form a self-sustaining flame or continuous burning.

2. Ignition temperature (autoignition temperature)
   The lowest temperature at which a substance can ignite on its own under specified conditions without an external flame (strongly dependent on test method). In engineering, distinguish:

• Flash point (vapor above a liquid can briefly ignite)
• Fire point (sustained burning)
• Autoignition point (no external ignition source)

Typical autoignition temperatures (engineering ranges):

• Methane: about $537^\circ C$
• Carbon monoxide: about $609^\circ C$
• Hydrogen: about $500\sim585^\circ C$ (condition-dependent)
  Solids (coal) are more affected by particle size, volatile content, moisture, and oxidation activity, so variation is larger.