The chassis is the basic structure of the vehicle. In a narrower sense, the chassis is only the support structure of the vehicle. In vehicle technology, though, what is often meant by the chassis is actually the frame (i.e. the support structure) complete with the suspension, steering and attachments that are directly fastened to the frame.
The so-called backbone tube chassis, which is used by a specialized manufacturer of heavy trucks and is supposed to provide particularly good off-road mobility, is a more exotic solution.
