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]