Plain English Meaning
The parts of the engine that create the spark needed to light the fuel and air inside each cylinder, timed precisely so the engine runs smoothly. Aircraft engines have two of these systems working at the same time for safety.
Full Definition
The ignition system in a reciprocating aircraft engine is the set of components that produces and delivers a high-voltage electrical spark to each cylinder's spark plugs at the correct moment to ignite the fuel/air mixture. In most piston aircraft engines it consists of two independent magnetos, two spark plugs per cylinder, ignition leads (wires), and the ignition switch. Each fires one in each cylinder, and the two systems operate independently of the aircraft's .