Lecithin is a phospholipid, a class of compounds that make up cell membranes throughout the body. It is produced in the liver if the diet contains enough of the raw ingredients.
Lecithin is widely used as an emulsifying agent, allowing oil and water to mix, as in mayonnaise. It is used in ice creams, salad dressings, cosmetics, and it is the main ingredient in non-stick spray coatings for cooking.
Lecithin is also an anti-oxidant, helping to keep fats from going rancid (going rancid itself in the process, however).
Lecithin is often taken as a dietary supplement, since it contains the B vitamin choline.
Lecithin is the emulsifier in egg yolks that allows the oil and water to mix to make mayonnaise.
beta lecithin: InChI=1/C44H88NO8P/c1-6-8-10-12-14-16-18-20-22-24-26-28-30-32-34-36-43(46)50-40-42(53-54(48,49)52-39-38-45(3,4)5)41-51-44(47)37-35-33-31-29-27-25-23-21-19-17-15-13-11-9-7-2/h42H,6-41H2,1-5H3