Definition from Wiktionary, the free dictionary
- The part of an internal combustion engine which forms a high-voltage electric spark which ignites the fuel-air mixture to begin the power stroke.
- The spark plug is quite simple in theory: It forces electricity to arc across a gap, just like a bolt of lightning. The electricity must be at a very high voltage in order to travel across the gap and create a good spark. Voltage at the spark plug can be anywhere from 40,000 to 100,000 volts. 
device that forms a high-voltage spark for fuel ignition