I think both Cheetah's and Yeezevee's replies are the typical descriptions of fire that we are educated with at school, but I think the typical description is more like an explanation of when fire occurs, rather than what fire is. That diagram above doesn't tell us anything about what fire is, just what it's a result of.
It does NOT tell us
"what fire is"......
interesting point J4m3z., words sounds like "Heisenberg Uncertainty Principle"
HUP states that
"precise inequalities that certain pairs of physical properties, such as position and momentum, cannot simultaneously be known to arbitrary precision"Any way talking about these 'Actions and detection"., in all these things detector is extremely important and one of the best detectors is Brain. That smart guy Christopher Eppig and his colleagues pubed a paper in the Proceedings of the Royal Society. It says
The human brain requires a prodigious amount of energy relative to its weight and size. A newborn baby uses 87 per cent of its metabolic energy for the development of its brain; this figure comes down to 44 per cent in a five-year old child. According to the Economist, an adult uses 25 per cent of his metabolic energy on his brain, even though this organ accounts for only two per cent of total body weight.

Log disability-adjusted life years owing to infectious disease and average national IQ correlate. Image credit:
Proceedings of the Royal Society B, Researchers in the US have noted areas of the world with the lowest average intelligence quotient (IQ) also tend to have the highest rates of infectious diseases, and suggest the energy required to fight off the diseases may hinder brain development in children because both are metabolically costly processes.
What a power in that small amount of matter. Without it., we see neither fire nor its effects..