Definition:
A "selsyn" is a type of system that includes a generator and a motor. These two parts are connected in such a way that the motor will move in the same way as the generator. This means that if the generator turns, the motor will turn to match it. They are synchronized, or working together in harmony.
In summary, a "selsyn" is a technical term used mainly in engineering to describe a specific type of system where a generator and a motor work together in sync.