lecithin

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

• noun Any of a group of phospholipids found in egg yolks and the plasma membrane of plant and animal cells, used as an emulsifier in a wide range of commercial products, including foods, cosmetics, paints, and plastics.

from The Century Dictionary.

• noun A nitrogenous fatty substance, to which the formula C₄₄H₉₀NPO₉ has been given, which is found in small quantity in the blood, bile, and other fluids of the body, but most abundantly in the brain- and nervetissues, in pus, and in the yolk of eggs.

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

• noun (Physiol. Chem.) A complex, nitrogenous phosphorized substance widely distributed through the animal body, and especially conspicuous in the brain and nerve tissue, in yolk of eggs, and in the white blood corpuscles.

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

• noun organic chemistry the principal phospholipid in animals; it is particularly abundant in egg yolks, and is extracted commercially from soy. It is a major constituent of cell membranes, and is commonly used as a food additive (as an emulsifier).

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

• noun a yellow phospholipid essential for the metabolism of fats; found in egg yolk and in many plant and animal cells; used commercially as an emulsifier