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 magneto fires one spark plug in each cylinder, and the two systems operate independently of the aircraft's electrical system.
Plain English
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.
Context Anchor
Seen in piston-aircraft engine starting, before-takeoff ignition checks, spark plug discussions, and troubleshooting engine roughness.
Derivation
From the Latin ignire, meaning 'to set on fire.' The system's job is exactly that — setting fire to the fuel/air mixture inside each cylinder.
Why Pilots Care
A faulty ignition system can prevent engine start or cause power loss in flight.
Intuition Check
Do not think of an aircraft ignition system as just a key switch like in a car. In many small aircraft, the main job is done by engine-driven spark generators, and the system can keep making spark even if the aircraft’s main electrical power is off.
Example Sentence 1
During the runup, the pilot checked the ignition system by switching from BOTH to L, then to R, watching for a small, even RPM drop on each magneto.
Example Sentence 2
A rough-running engine after takeoff often points to an ignition system problem.