A Scooter is a two-wheeled motor vehicle with a step-through frame.
Historically, a scooter was noted for a small engine, small wheels, a step-through design, a forward fairing with floorboards, and possibly under-seat storage. It is important to note that the definition of scooter continues to evolve however -- even from a period in the early 1960's, when large-wheeled scooters were first introduced.
Modern scooters cover a broad spectrum of cycle design: step-through design or step-over design, small or large wheels, front fairings or floor boards, under-seat storage or not, manual or automatic transmissions -- though most scooters today do feature automatic transmissions.
At one end of the current market, the Vespa LX series reflects the scooters' historical antecedents: small-wheeled, floor boards, front fairing, inner fairing storage. At the other end of the spectrum, the Honda Big Ruckus featured no bodywork, floorboards and no hallmark "step-through" -- though still was classified as a scooter. The Piaggio MP3, with two front wheels (three wheels total); reflects the fluid nature of the scooter classification. Likewise, modern scooters have a wide range of engine displacements from under 50cc to over 799cc -- and some have engine locations in stark contrast classic scooter design (e.g. Yamaha T-Max 500, Suzuki Burgman 650, Gilera GP800);.
Furthermore, legal definitions of a scooter vary from country to country, locality to locality. For example, the US Department of Transportation defines 'scooters' separately from motorcycles in order to regulate their compliance with various standards -- notably locations of throttle and braking controls, which differ considerably from motorcycle controls. Furthermore, in some areas of the US, a scooter must have an engine size above 49cc otherwise it is classed as a moped -- thereby falling into different legal status. In Europe, scooters less than 125cc's have special tax and licensing benefits.