Source : Webster's Revised Unabridged Dictionary (1913)

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}.

         I had my father's signet in my purse.    --Shak.

   {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.

Source : WordNet®

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