The Collaborative International Dictionary of English v.0.48:

Trade-mark \Trade"-mark`\, n.
   A peculiar distinguishing mark or device affixed by a
   manufacturer or a merchant to his goods, the exclusive right
   of using which is recognized by law.
   [1913 Webster]

WordNet (r) 3.0 (2006):

trademark
    n 1: a distinctive characteristic or attribute [syn: hallmark,
         trademark, earmark, stylemark]
    2: a formally registered symbol identifying the manufacturer or
       distributor of a product
    v 1: mark with a brand or trademark; "when this product is not
         branded it sells for a lower price" [syn: brand,
         trademark, brandmark]
    2: register the trademark of; "The company trademarked their new
       gadget"

Moby Thesaurus II by Grady Ward, 1.0:

59 Moby Thesaurus words for "trademark":
   acknowledgment, billhead, book stamp, bookplate, brand,
   broad arrow, by-line, cachet, certificate of invention, check,
   citation, colophon, confession, copyright, counterfoil,
   countermark, credit line, docket, government mark,
   government stamp, habit, hallmark, idiosyncrasy, imprint, label,
   letterhead, logo, logotype, mannerism, masthead, minauderie,
   patent, peculiar trait, peculiarity, plate, price tag, quirk,
   reference, registered trademark, running head, running title, seal,
   service mark, sigil, signature, signet, stamp, sticker, stub, tag,
   tally, ticket, title page, token, trade name, trademark name,
   tribute, trick, trick of behavior