Definitions really depend on the state. See below for VA:
Moped:
"Moped" means every vehicle that travels on not more than three wheels in contact with the ground that (i) has a seat that is no less than 24 inches in height, measured from the middle of the seat perpendicular to the ground; (ii) has a gasoline, electric, or hybrid motor that (a) displaces 50 cubic centimeters or less or (b) has an input of 1500 watts or less; (iii) is power-driven, with or without pedals that allow propulsion by human power; and (iv) is not operated at speeds in excess of 35 miles per hour.
"Moped" does not include an electric power-assisted bicycle or a motorized skateboard or scooter. For purposes of this title, a moped shall be a motorcycle when operated at speeds in excess of 35 miles per hour. For purposes of Chapter 8 (§
46.2-800 et seq.), a moped shall be a vehicle while operated on a highway.
Electric power-assisted bicycle:
"Electric power-assisted bicycle" means a vehicle that travels on not more than three wheels in contact with the ground and is equipped with (i) pedals that allow propulsion by human power, (ii) a seat for the use of the rider, and (iii) an electric motor with an input of no more than 750 watts. Electric power-assisted bicycles shall be classified as follows:
1. "Class one" means an electric power-assisted bicycle equipped with a motor that provides assistance only when the rider is pedaling and that ceases to provide assistance when the bicycle reaches a speed of 20 miles per hour;
2. "Class two" means an electric power-assisted bicycle equipped with a motor that may be used exclusively to propel the bicycle and that ceases to provide assistance when the bicycle reaches the speed of 20 miles per hour; and
3. "Class three" means an electric power-assisted bicycle equipped with a motor that provides assistance only when the rider is pedaling and that ceases to provide assistance when the bicycle reaches the speed of 28 miles per hour.
For the purposes of Chapter 8 (§
46.2-800 et seq.), an electric power-assisted bicycle shall be a vehicle when operated on a highway.
Bicycle:
"Bicycle" means a device propelled solely by human power, upon which a person may ride either on or astride a regular seat attached thereto, having two or more wheels in tandem, including children's bicycles, except a toy vehicle intended for use by young children. For purposes of Chapter 8 (§
46.2-800 et seq.), a bicycle shall be a vehicle while operated on the highway.
Virginia Code Link