A commercial vehicle is defined in Part 390.5 of the FMCSR as:
Any self propelled or towed motor vehicle operated in commerce, and;

Has a gross vehicle weight rating (GVWR), or a gross combination weight rating
(GCWR), or a gross vehicle weight, or a gross combination weight of 10,001
pounds or more, or;

Is designed or used to transport more than 8 passengers (including the driver) for
compensation, or;

Is designed or used to transport more than 15 passengers (including the driver)
and is not used to transport passengers for compensation, or;

A vehicle, regardless of weight, used to transport a type or quantity of hazardous
materials required to be placarded.

Also, there are some exceptions in state statute for some types of vehicles, such as solid
waste haulers and farm licensed vehicles. There is an exception for medical certificate
requirements for some drivers.