What's the difference between chassis and a cabinet for a guitar amp? Are there any concrete definitions?
The chassis is the metal box that all the electronic components are housed in - valves, their seats, pcbs, pots, etc. The part seen at the front/top of the amp. where the control knobs sit. Some amps have two chassis, often one for the pre amp and the other for the power amp.
The cabinet is very different. Usually made from wood (ply) and covered in carpet or plastic, housing the speaker/s and sometimes the amp. as well. It can be open or closed backed.
An amp may consist of a smaller cabinet just around the chassis, or may have a speaker inside the cabinet too. That's known as a combo. Or the cabinet may just contain speaker(s) with/without a crossover, in which case it needs a separate head (to act as the amp) in its chassis in its cabinet. Though in this case, the cabinet has no sonic influence; it's function is just to protect the amp and its components.