Autosynchronous Motor

It's an old old term, but still gets used sometimes.

A long time ago, synchronous motors were just synchronous motors, but were inherently incapable of starting on their own. Basically, it was really a synchronous GENERATOR being used as a motor. If you think of a generator, it has a "prime mover", some sort of external force such as a water or steam turbine that accelerates it and pushes it. But as a motor, they had to be started with what was called a "pony motor" which was a smaller DC or AC induction motor that only accelerated the mass of the Synchronous motor itself up to the proper speed. Then when at the right speed, the synchronous motor would be energized and "pull in" to synchronism with the line frequency, after which a clutch would engage the load (if any).

Synchronous machines have a dampening feature, called an "amortissuer", in the rotor to decrease pulsations in the machine when the load attempts to pull it out of synchronicity. The design of that amortissuer is essentially the same as the rotor bars in an induction (asynchronous) motor. So eventually some smart people discovered that if you energize the stator without the rotor field, the amortissuer will act like an induction motor to accelerate the rotor to the slip speed, which is close enough to synchronous that applying the rotor field to it at that point makes it pull in. So by doing that, they referred to those as "auto-synchronous" motors because they accelerated themselves and thus did not need the pony motor. But really, it's the same, it's just that people didn't think of using the amortissuer that way at first.