A state in which thermonuclear fusion generates all the thermal energy needed to maintain fusion conditions, thus eliminating the need for external heating. The state can be compared to a fire burning in a fireplace - it generates enough heat to continue burning. The “ignition” state is estimated to occur for Q > 5, i.e. when the fusion starts to produce at least five times as much energy as is required to sustain it. Only after ignition has been reached can the fusion technology be used as an energy source in a fusion power plant.

The term “ignition” is sometimes used to refer to the fact that a fusion reaction has taken place, regardless of how much energy it produced.