The Collaborative International Dictionary of English v.0.48:

Initials \In*i"tials\, n. pl.
   The first letters of a person's first and last name, and
   sometimes also the first letters of the middle name or names;
   as, sign your initials in the margin; people identified only
   by their initials.
   [PJC]

Moby Thesaurus II by Grady Ward, 1.0:

39 Moby Thesaurus words for "initials":
   ID card, John Hancock, X, autograph, badge, calligram,
   calling card, card, christcross, cipher, countermark, countersign,
   countersignature, counterstamp, credentials, cross, device,
   dog tag, endorsement, hand, identification, identification badge,
   identification tag, letter of introduction, mark,
   mark of signature, monogram, press card, seal, serial number,
   sigil, sign manual, signature, signet, subscription, tessera, visa,
   vise, visiting card