Wordnet 3.0
NOUN (2)
1. 
 military identification tag worn on a chain around the neck; 
2. 
 metal plate on a dog collar bearing its registration number; 
WordNet (r) 3.0 (2006):
dog tag
    n 1: military identification tag worn on a chain around the neck
    2: metal plate on a dog collar bearing its registration number
Moby Thesaurus II by Grady Ward, 1.0:
19 Moby Thesaurus words for "dog tag":
   ID card, badge, calligram, calling card, card, countermark,
   countersign, credentials, identification, identification badge,
   identification tag, initials, letter of introduction, monogram,
   press card, serial number, signature, tessera, visiting card