dog tag

American Heritage® Dictionary of the English Language, Fourth Edition

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

Wiktionary

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

WordNet 3.0

1. n. metal plate on a dog collar bearing its registration number
2. n. military identification tag worn on a chain around the neck