Plain English Meaning
A device that keeps all the propellers spinning at the same speed automatically, so the pilot does not have to fine-tune each engine by hand.
Full Definition
A system installed on multi-engine aircraft that automatically matches the rotational speed (RPM) of all propellers to a single chosen engine, called the master engine. The pilot sets the desired RPM on the master engine, and the synchronizer adjusts the governor on each slave engine to hold its propeller at exactly the same speed.
Context Anchor
Pilots encounter this during multi-engine operation, especially in cruise after the power and propeller speed settings have been set.
Derivation
Synchronizer comes from the Greek 'synchronos,' meaning 'happening at the same time.' In this case, it keeps the propellers turning at the same time at the same speed.