Is an engine's bore a measurement of the combustion chamber or the piston?

Is an engine's bore a measurement of the combustion chamber or the piston?
The cylinder. When you "Bore Out" an engine block, you're making the combustion chamber wider to accommodate larger pistons to increase horsepower. That's usually mostly done in drag racers, not much use for it in a street car, unless you like supportin' the local Policemans Ball.