NOUN (1)

1. 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;

The Collaborative International Dictionary of English v.0.48:

Lecithin \Lec"i*thin\, n. [Gr. le`kiqos the yolk of an egg.]
   (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.
   [1913 Webster]