saponin

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of various plant glycosides that form soapy lathers when mixed and agitated with water, used in detergents, foaming agents, and emulsifiers.

from The Century Dictionary.

• noun A glucoside (C₃₂H₅₄O₁₈) found in the root of Saponaria officinalis and many other plants. It is a powerful sternutatory.
• noun A general name applied to glucosides similar to saponin (see def. 1) which yield a foam or lather when the aqueous solution is shaken. Smilacin is a saponin. The poisonous saponins are called sapotoxins.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Chem.) 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 anæsthesia. 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun organic chemistry, biochemistry Any of various steroid glycosides found in plant tissues that dissolve in water to give a soapy froth.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

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