A modified, customized motorcycle, typically a Harley Davidson, although "back in the day" Indian, Triumph and BSA motorcycles were "chopped" and these days Japanese motorcycles are being transformed into choppers.
The term "chopper" comes from the origins of motorcycle customization. Unnecessary parts were (sometimes literally) "chopped" off to reduce the weight of the motorcycle and increase its performance for racing. Rear fenders were "bobbed" (where the term "bobber" comes from), front fenders, crash bars, saddlebags, windshields, head and tail lights, kickstands, mufflers, etc. were all discarded to improve the machine's power-to-weight ratio. Forks were extended to improve ground clearance and later raked to compensate for better stability.
Contrary to popular belief, a chopper is not simply a motorcycle built with long forks, a stretched frame, chrome and billet crap and a gazillion-color $10K paintjob. True choppers are generally built from another motorcycle or motorcycles, by the owner of the motorcycle and modifications are done to the builder/owner's desires and usually done on a budget.
The big-bucks "chopper" shops do not build choppers. OCC does not build true choppers. They build custom motorcycles. A custom can emulate the classic chopper styles (bobber, East Bay lowrider, digger, fat bob, etc.) but it is not a true chopper. Same as a fibreglass-bodied 350/350-powered 5-window coupe that looks like a hot rod, it is not a true hot rod.
Whose motorcycle is this?
It's a chopper, baby.
Whose chopper is this?
Zed's dead, baby. Zed's dead.