1. A yellow phospholipid (any of various compounds composed of fatty acids and phosphoric acid and a nitrogenous base; an important constituent of membranes) essential for the metabolism of fats; found in egg yolk and in many plant and animal cells; used commercially as an emulsifier.
2. 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.