Fusion ignition is the point at which a nuclear fusion reaction becomes self-sustaining. This occurs when the energy being given off by the reaction heats the fuel mass more rapidly than it cools. In other words, fusion ignition is the point at which the increasing self-heating of the nuclear fusion removes the need for external heating.
This is quantified by the Lawson criterion.
Ignition can also be defined by the fusion energy gain factor.
In the laboratory, fusion ignition defined by the Lawson criterion was first achieved in August 2021,
and ignition defined by the energy gain factor was achieved in December 2022,
both by the U.S. National Ignition Facility.