A poisonous glucoside found in many plants, as in the root
   of soapwort (Saponaria), in the bark of soap bark (Quillaia), etc. It
   is extracted as a white amorphous powder, which occasions a soapy
   lather in solution, and produces a local anaesthesia. Formerly called
   also struthiin, quillaiin, senegin, polygalic acid, etc. By extension,
   any one of a group of related bodies of which saponin proper is the
   type.