Note there is no single fixed definition of a scooter (also known by the full name motor-scooter), but generally a smaller motorcycle with a step-through frame is considered a scooter, especially if it has a floor for the rider's feet (as opposed to straddling the vehicle like a conventional motorcycle).
Other common traits of scooters can include: bodywork (so the mechanicals are not exposed like a conventional motorcycle), motors combined with the suspension or wheel (rather than attached to the frame like a conventional motorcycle), leg shields, smaller wheels than a conventional motorcycle, and an alternative to a chain drive.
[1] Scooters share traits with mopeds (some models could even be considered both a moped and a scooter).
Adding to the confusion between them, in many jurisdictions smaller engined scooters (e.g. 50cc) are road registered in the same legal category as mopeds (often named "Moped" class), leading to scooters being casually referred to as "mopeds" in such areas.
[2] Underbones also share traits with scooters (e.g. small and step-through design), but they are generally not strictly considered scooters in the purest sense as they do not have a floor, but they are often casually referred to as scooters (especially ones with leg shields).