license plate

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A rectangular, usually metal plate that bears a sequence of numbers, letters, or both and is issued by a government to identify an officially registered vehicle.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun a tag, usually of metal, bearing a sequence of letters, numbers or both, and placed at the front and back of a motor vehicle, as for identification purposes

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a plate mounted on the front and back of car and bearing the car's registration number