Definition
A sealed mechanical housing containing a set of gears that transmits power from one rotating shaft to another, typically changing the speed, torque, or direction of rotation. In aircraft, gear-boxes are used to drive accessories such as magnetos, alternators, fuel pumps, vacuum pumps, and propellers from the engine's crankshaft, and to step engine speed up or down to match the required output speed of those components.
Plain English
A box full of gears that takes the spinning motion from the engine and passes it on to other parts at a different speed or in a different direction.
Context Anchor
Seen in engine system discussions, especially where engine power is carried to a propeller or other engine-driven part.
Derivation
Plain English: a 'box' that holds 'gears.' The hyphenated form 'gear-box' is the older spelling; modern usage is usually 'gearbox.' Knowing the literal meaning helps — it really is just an enclosed case of meshed gears doing one job: changing rotational speed or direction.
Why Pilots Care
Allows the engine to run at high RPM for maximum power output while keeping propeller RPM within safe structural and aerodynamic limits.
Intuition Check
Gear-box does not mean landing gear, and it is not just a storage box. Here it means the enclosed gear mechanism that carries engine power to another part.
Example Sentence 1
On turboprop engines, a reduction gear-box slows the high engine speed down to a usable propeller speed.
Example Sentence 2
A failure in the reduction gearbox would prevent the propeller from turning at the correct speed for takeoff.