The act of exiting one side of a moving automobile (usually through the window, as a door catches the wind and can slam on you), climbing over the bonnet or boot (or the roof, if you're confident) and re-entering the vehicle on the other side. Usually this is done without any safety equipment, but if the car is also equipped for car surfing, there are usually ropes present. Don't decide to be clever and press your face against the windshield, as the driver may turn the windscreen wipers on, and those things actually hurt a bit. Typically, alcohol helps to convince people that getting out of a moving car and getting back in on the opposite side is a good idea, but it can also be a useful way to change seats without stopping.

Also (originally) the term used to describe an astronaut (or cosmonaut) exiting their spacecraft and traversing the outside of it. They can be tethered (attached with a string type affair, only more hi-tech) or untethered, and they may or may not have some sort of rocket pack type affair.