Definition
On a multiengine airplane equipped with a propeller synchronization system, the master engine is the engine whose propeller speed is used as the reference. The synchronizer measures the rpm of the master engine's propeller and automatically adjusts the other propeller(s) to match it.
Plain English
The engine the synchronizer uses as the standard. Its propeller sets the speed, and the other propeller is brought into step with it.
Context Anchor
Seen in multi-engine airplane operations when using a propeller synchronization system to reduce propeller speed mismatch between engines.
Derivation
Master' here carries its mechanical/control sense -- the part that leads, with others following. Same idea as a master cylinder in brakes or a master switch in electrical systems: one component sets the reference and the rest respond to it.
Why Pilots Care
Setting one RPM on the master engine lets the system keep all propellers turning at identical speeds, which reduces cabin noise and vibration and extends propeller and engine life.
Intuition Check
Master does not mean the engine produces more power or is more important for flight. Here it means the engine whose propeller speed is used as the reference for matching the other propeller.
Example Sentence 1
After leveling off in cruise, the pilot set the master engine's propeller to 2300 rpm and switched on the synchronizer to bring the other propeller into step.
Example Sentence 2
Once the master engine was stabilized, the synchronization system brought the right propeller into exact RPM match without further pilot input.