[1] : The Collaborative International Dictionary of English v.0.48

saponin \sap"onin\, 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
see also:
saponaria officinalis quillaja saponaria struthiin quillaiin senegin polygalic acid 

[2] : WordNet (r) 2.0

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

Saponin >> S Words
Saponin, definition of term: Saponin
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