The moment the fire begins, when fuel, air, and an ignition source all come together...

The ignition stage is the point at which a fire starts. This occurs when fuel, heat, and oxygen are present in the correct combination and for the right duration. It marks the beginning of combustion and transitions the scenario from a fire hazard to an active fire.

Free burning (A) is a later stage where flames are fully developed.

Pyrolysis stage (C) refers to pre-ignition gas release.

Incipient stage (D) can be synonymous with ignition but more commonly refers to the pre-flame development phase.