OLDTIMER...each state is different but I am speaking NY state law...operate a motor vehicle means a motorized vehicle(car,bus,truck,boat,snowmobile)...interesting bit about NY-if you have been drinking and pull over in a rest stop to sleep it off and have the car on you are operating a motor vehicle and can be arrested...do a simple thing like take the key out of the ignition and go to sleep-you cannot be arrested for operating a mototr vehicle under the influence....each state is a little different so be more specific when asking
in most states, if you're drunk in the car, even if the keys are out of the ignition and in your pocket, you can get a DUI. (you're sleeping in the car, legally drunk, at .08 or higher, and the keys are in the glovebox, you'll get a DUI.... if you sleep in your car, toss the keys outside or under the car, and hope you find them in the morning)
in CA, if the keys are in the ignition and you're OUTSIDE the car, you can get a DUI (as well as other states).