Saponin

from WordNet (r) 3.0 (2006)

saponin
    n 1: any of various plant glucosides that form soapy lathers
         when mixed and agitated with water; used in detergents and
         foaming agents and emulsifiers

from The Collaborative International Dictionary of English v.0.48

Saponin \Sap"o*nin\, n. [L. sapo, -onis soap: cf. F. saponine.]
   (Chem.)
   A poisonous glucoside found in many plants, as in the root of
   soapwort ({Saponaria officinalis}), in the bark of soap bark
   ({Quillaja saponaria}), etc. It is extracted as a white
   amorphous powder, which produces a soapy lather in solution,
   and produces a local anaesthesia. It is used as a detergent
   and for emulsifying oils. 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.
   [1913 Webster + PJC]