RCW 46.04.304: Moped.

"Moped" means a motorized device designed to travel with not more than three wheels in contact with the ground and having an electric or a liquid fuel motor with a cylinder displacement not exceeding fifty cubic centimeters which produces no more than two gross brake horsepower (developed by a prime mover, as measured by a brake applied to the driving shaft) that is capable of propelling the device at not more than thirty miles per hour on level ground.