A
metal plate that is
required by law in every
country in the
world to be
attached to the
back of a
motor vehicle (and also the
front in some of those places). This plate contains a
unique identifier, allowing each
vehicle to be
tracked for whatever purposes, including
bad driving.
Some places make them decorative, or at least try to, while others make them plain and simple.
The identifier on the place is often called the license plate number.