VEH § 630

A “trailer” is a vehicle designed for carrying persons or property on its own structure and for being drawn by a motor vehicle and so constructed that no part of its weight rests upon any other vehicle. As used in Division 15 (commencing with Section 35000), “trailer” includes a semitrailer when used in conjunction with an auxiliary dolly, if the auxiliary dolly is of a type constructed to replace the function of the drawbar and the front axle or axles of a trailer.