Electric powered attachments specifically designed for wheelchair use. They consist of a single drive wheel which is attached to the user's wheelchair by a docking mechanism of various types.
These are the "classic" scooters that balance on two wheels. They are the most common type of scooter and are widely available throughout the world at various price points.
There are two styles of three wheel scooters - Tricycle (1 in front, 2 in rear) and Reverse-Tricycle (2 in front, in rear) which lead to different performance characteristics.