Plain English Meaning
A device that keeps all the propellers spinning at exactly the same speed so the engines sound and feel smooth instead of producing that wobbling, throbbing noise you sometimes hear in twin-engine airplanes.
Full Definition
A system on multi-engine airplanes that automatically matches the rotational speed (RPM) of all propellers to a single reference, eliminating the slight RPM differences between engines that cause a pulsing beat sound and vibration in the cabin.
Context Anchor
Encountered in multiengine airplane operation, especially after power and propeller controls are set for cruise flight.
Derivation
From Latin synchronus, meaning 'happening at the same time' (syn- 'together' + chronos 'time'). The system makes the propellers turn together in time.