The Collaborative International Dictionary of English v.0.48:

Signet \Sig"net\, n. [OF. signet a signet, F., a bookmark, dim.
   of signe. See Sign, n., and cf. Sennet.]
   A seal; especially, in England, the seal used by the
   sovereign in sealing private letters and grants that pass by
   bill under the sign manual; -- called also privy signet.
   [1913 Webster]

         I had my father's signet in my purse.    --Shak.
   [1913 Webster]

   Signet ring, a ring containing a signet or private seal.

   Writer to the signet (Scots Law), a judicial officer who
      prepares warrants, writs, etc.; originally, a clerk in the
      office of the secretary of state.
      [1913 Webster]

WordNet (r) 3.0 (2006):

signet
    n 1: a seal (especially one used to mark documents officially)

Moby Thesaurus II by Grady Ward, 1.0:

138 Moby Thesaurus words for "signet":
   John Hancock, OK, X, acceptance, affirmance, affirmation,
   approbation, approval, authentication, authorization, autograph,
   billhead, book stamp, bookplate, boss, brand, broad arrow, bump,
   cachet, cap of dignity, cap of maintenance, certification, check,
   christcross, cipher, colophon, concavity, confirmation, convexity,
   coronet, counterfoil, countermark, countersign, countersignature,
   counterstamp, cross, crown, dactylogram, dactylograph, dent,
   device, diadem, dint, docket, embossment, endorsement, ermine,
   excrescence, fingerprint, footmark, footprint, footstep,
   fossil footprint, go-ahead, government mark, government stamp,
   great seal, green light, hallmark, hand, ichnite, ichnolite,
   impress, impression, imprimatur, imprint, indent, indentation,
   indention, initials, label, letterhead, logo, logotype, lump, mark,
   mark of signature, masthead, monogram, nod, notarization, okay,
   orb, pad, paw print, pawmark, permission, pimple, plate, price tag,
   print, privy seal, pug, pugmark, purple, purple pall, ratification,
   regalia, registered trademark, robe of state, rod, rod of empire,
   royal crown, rubber stamp, running head, running title, sanction,
   scepter, seal, sigil, sign manual, signature, stamp,
   stamp of approval, step, sticker, stub, stud, subscription, tag,
   tally, the nod, thumbmark, thumbprint, tiara, ticket, title page,
   token, trade name, trademark, trademark name, triple plume, uraeus,
   validation, vestige, visa, vise, warrant



Easton's 1897 Bible Dictionary:

Signet
   a seal used to attest documents (Dan. 6:8-10, 12). In 6:17, this
   word properly denotes a ring. The impression of a signet ring on
   fine clay has recently been discovered among the ruins at
   Nineveh. It bears the name and title of an Egyptian king. Two
   actual signet rings of ancient Egyptian monarchs (Cheops and
   Horus) have also been discovered.

     When digging a shaft close to the south wall of the temple
   area, the engineers of the Palestine Exploration Fund, at a
   depth of 12 feet below the surface, came upon a pavement of
   polished stones, formerly one of the streets of the city. Under
   this pavement they found a stratum of 16 feet of concrete, and
   among this concrete, 10 feet down, they found a signet stone
   bearing the inscription, in Old Hebrew characters, "Haggai, son
   of Shebaniah." It has been asked, Might not this be the actual
   seal of Haggai the prophet? We know that he was in Jerusalem
   after the Captivity; and it is somewhat singular that he alone
   of all the minor prophets makes mention of a signet (Hag. 2:23).
   (See SEAL.)