The frame underneath a car, truck, or other vehicle — the strong metal skeleton that everything else (the engine, the body, the seats) is built onto. Mechanics and car fans use this word a lot; most other people only need it when something serious happens to a car, like after a bad accident, when you might hear 'the chassis is bent.' It can also be used for the frame inside other machines, like a piece of electronic equipment, though that's less common.