Refers to a motor scooter (Vespa-style) or sometimes a kick scooter, depending on context. In Japanese, context usually makes the type clear.
He commutes to work every day by scooter.
The children were playing on their scooters in the park.
I commute to work on a scooter every day.
You need a license to drive a scooter.
I parked my scooter in front of the station.