Definition of "spark plug"
spark plug
noun
plural spark plugs
(military) A rod (for cylindrical secondaries) or sphere (for spherical secondaries) of fissile material at the center of the secondary of a multistage thermonuclear weapon, which undergoes nuclear fission when compressed to supercriticality by the imploding secondary, helping to raise the secondary to the pressures and temperatures necessary to ignite nuclear fusion in the secondary's fusion fuel.