Source : Webster's Revised Unabridged Dictionary (1913)

Tetradrachm \Tet"ra*drachm\, Tetradrachma \Tet`ra*drach"ma\, n.
   [NL. tetradrachma, fr. Gr. tetra`drachmon; te`tra- (see
   {Tetra-}) + drachmh` drachm, drachma.]
   A silver coin among the ancient Greeks, of the value of four
   drachms.

   Note: The Attic tetradrachm was equal to 3s. 3d. sterling, or
         about 76 cents.