Definition
An engine-driven electrical generator that produces high-voltage current to fire the spark plugs in a piston aircraft engine, using a rotating magnet and coil rather than the airplane's battery or electrical system. Most certified piston aircraft have two independent magnetos (left and right) for redundancy, each firing one of the two spark plugs in every cylinder.
Plain English
A small generator on the engine that makes its own electricity to create the sparks that ignite the fuel. It works on its own, without needing the airplane's battery, so the engine keeps running even if the electrical system fails.
Context Anchor
You encounter magnetos during preflight and before-takeoff checks, especially when checking the ignition switch positions and listening for normal engine operation.
Derivation
From 'magneto-electric machine,' shortened to 'magneto.' The name comes from the device's use of a spinning magnet to generate electricity. Knowing it is magnet-driven helps explain why it keeps working without battery power -- the engine itself spins the magnet.
Why Pilots Care
Each magneto supplies ignition to one set of spark plugs; a failed magneto or improper grounding during preflight can cause rough running or engine stoppage.
Analogy
Think of a bicycle dynamo that powers a headlight from the turning wheel. The magneto works the same way, turning engine rotation into the high-voltage spark needed for combustion.
Intuition Check
Do not think of a magneto as the airplane’s battery. A magneto makes ignition electricity for the engine, and it can do that independently of the aircraft’s normal electrical power.
Example Sentence 1
During the run-up, the pilot switched from BOTH to L, noted the RPM drop, then back to BOTH before checking the right magneto.
Example Sentence 2
At engine run-up the pilot switched from both magnetos to the left magneto alone and noted a small drop in rpm.