1.NounThe part of some internal combustion engines which forms a high-voltage electric spark which ignites the fuel-air mixture to begin the power stroke. (Petrol engines have spark plugs. Diesel engines do not have them because the heat from their high compression ratios supplies the ignition.)
"In the 21st century, it is not unusual for a set of spark plugs to last a decade or longer; gone are the days when changing the plugs annually was common."
2.NounSomeone who is a driving force in new endeavours.
"Helen is a real spark plug. We're lucky she's on the team."
3.NounA 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.