The Collaborative International Dictionary of English v.0.48:

Hall-mark \Hall"-mark`\ (h[add]l"m[aum]rk`), n.
   1. The official stamp of the Goldsmiths' Company and other
      assay offices, in the United Kingdom, on gold and silver
      articles, attesting their purity.
      [1913 Webster]

   2. Hence, [figuratively]: A distinguishing characteristic or
      characteristics; as, a word or phrase lacks the hall-mark
      of the best writers.
      [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

hallmark
    n 1: a distinctive characteristic or attribute [syn: hallmark,
         trademark, earmark, stylemark]
    2: a mark on an article of trade to indicate its origin and
       authenticity [syn: authentication, hallmark, assay-
       mark]

Moby Thesaurus II by Grady Ward, 1.0:

105 Moby Thesaurus words for "hallmark":
   aroma, attribute, authentication, badge, banner, billhead,
   book stamp, bookplate, brand, broad arrow, cachet, cast, character,
   characteristic, check, colophon, configuration, counterfoil,
   countermark, cut, device, differentia, differential,
   distinctive feature, docket, earmark, feature, figure, flavor,
   government mark, government stamp, gust, identification, idiocrasy,
   idiosyncrasy, image, impress, impression, imprint, index, indicant,
   indicator, individualism, insignia, keynote, label, letterhead,
   lineaments, logo, logotype, mannerism, mark, marking, masthead,
   measure, mold, nature, note, odor, particularity, peculiarity,
   picture, plate, price tag, property, quality, quirk,
   registered trademark, representation, representative, running head,
   running title, savor, seal, shape, sigil, sign, signal, signature,
   signet, singularity, smack, specialty, stamp, sticker, stub,
   sure sign, symbol, symptom, tab, tag, taint, tally, tang, taste,
   telltale sign, ticket, title page, token, trade name, trademark,
   trademark name, trait, trick, verification