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 ring
    n 1: a ring bearing a signet [syn: signet ring, seal ring]