The definition of “bicycle” varies by State, but it generally:

• Excludes all vehicles with motors.
• Excludes push vehicles without pedals (“scooters”).
• Often includes adult tricycles.
• Sometimes excludes children's toy bicycles. In some States this exclusion is based on wheel size - which may have the unintended consequence of excluding adult small-wheel bicycles, such as recumbent and folding bicycles.

Motorized bicycles are usually separately defined and are subject to some different rules.