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]

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