Plain English Meaning

It is a small tank of pressurized oil that helps move a feathered propeller back to a normal blade angle so the engine can be restarted in flight.

Full Definition

A small pressurized oil reservoir, fitted to some feathering propeller systems, that stores engine oil under pressure while the engine is running. When the pilot moves the propeller control out of the feather position to restart a feathered engine in flight, the accumulator releases its stored oil to the propeller hub, driving the blades out of the feathered position so the propeller can begin to windmill and the engine can be restarted.