What is the legal definition of a "-trailer"?

1. A motorized trailer with a frontal part that rests on a towing vehicle.
2. A motor vehicle that is designed to be towed by another vehicle.
3. Correct answer: A trailer that has its front part resting on the support surface behind a cab.
4. A trailer that has its rear part supported by a cab.

WhyA semi-trailer is a trailer whose front part rests on the support surface behind a cab. It has no front wheels of its own.