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]

WordNet (r) 3.0 (2006):

lecithin
    n 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