dog tag

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

• noun A metal identification disk attached to a dog's collar.
• noun A metal identification tag worn on a chain around the neck by members of the armed forces.

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

• noun A small metal disk, attached to a dog's collar, containing identification information
• noun by extension The identification tags worn around the neck by military personnel

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

• noun metal plate on a dog collar bearing its registration number
• noun military identification tag worn on a chain around the neck