Definition
A set of gears that reduces the high rotational speed of a turbine or engine output shaft to a lower speed suitable for driving the propeller. The gears step the rpm down so the propeller turns at an efficient speed while the engine continues to operate at its much higher design speed.
Plain English
A gearbox that slows down the spinning from the engine before it reaches the propeller, so the propeller turns at a sensible speed even though the engine is spinning much faster.
Context Anchor
Seen in turboprop and free-turbine engine discussions, especially when explaining how the engine drives the propeller.
Derivation
From 'reduction' (to make less) and 'gearing' (a system of gears). The name simply describes what the gears do: reduce speed. Gears with different tooth counts trade speed for torque, so a smaller drive gear turning a larger driven gear slows the output and increases turning force.
Why Pilots Care
Allows high-speed turbines to power slower propellers without damage or inefficiency.
Analogy
Like the gears on a bicycle: the pedals (engine) can spin at a comfortable rate while the wheel (propeller) turns at a different speed suited to the road.
Intuition Check
Reduction gearing does not mean the engine is being weakened. It means the gear system reduces shaft speed so the propeller can turn at the right speed.
Example Sentence 1
The turboprop's reduction gearing allows the turbine to spin at over 30,000 rpm while the propeller turns at around 2,000 rpm.
Example Sentence 2
Free-turbine engines use reduction gearing so the propeller can be feathered independently of the gas generator section.