In this article, unless the context otherwise requires:
1. "Envelope" means the outermost dimensions of a load or vehicle to include width, height and length and the weight of the vehicle or its load.
2. "Envelope permit" means a permit issued by the department authorizing a person to transport property on the highways of this state pursuant to all applicable statutes and local ordinances and the terms listed on the permit.
3. "Power unit" means a vehicle that propels itself or tows a trailer or combination of vehicles on the highways of this state.