ignition

noun

Etymology: From French ignition.

1. The act of igniting.
2. The initiation of combustion.
3. A system for activating combustion in a combustion engine.
4. Receptacle for a key used to start the engine in a vehicle.

   “Put the key in the ignition and turn in right.”

5. The phenomenon of a thermonuclear fusion reaction becoming self-sustaining and no longer requiring external heating.