In a car, an immobilizer is a hidden piece of equipment — often tied to a chip in the key — that stops the engine from running if someone tries to steal the car without the right key. The thief may open the door, but the car will not start, or it will die at once. Mechanics, insurers, and garages use this word; drivers often just talk about a chip key. British spelling is often immobiliser. Though actually, hospitals also use immobilizer for a brace that keeps a broken limb still — same idea of 'stop it moving', different object. This entry is the car kit, not the hospital brace. It is technical, everyday among people who work on cars, and a little specialised for everyone else.