noun
1. : a part that fits into the cylinder head of an internal-combustion engine and carries two electrodes separated by an air gap across which the current from the ignition system discharges to form the spark for combustion
2. : one that initiates, affords the impetus to, or is the most effective or dynamic force in an undertaking or work : prime mover , mainspring
supervisors are the spark plugs in any program of vocational education — E.A.Lee
his responsibility to act as spark plug when his team is in the field — C.F.Stubblefield
the profit motive as a spark plug of progress — S.T.Williamson