Joyriding is taking a car that is not yours and driving it for the thrill — often fast, often at night, often with friends — not because you want to keep the car or sell it. The law still treats this as a crime, even if the driver later leaves the car on a street. The heart of the word is the ride, not stealing a car as property. People do stretch it: a parent might laugh that the teenagers were joyriding around town in the family car, with permission. That is not the crime; that is just driving about for fun. You will hear the serious use a lot in British news. American news says it too, though it may simply say the car was stolen. Though actually, taking a car to keep it, strip it, or sell it is a different kind of wrong in people's minds — joyriding is about the fun of driving someone else's car, then often dumping it.