Essential fatty acids are fatty acids needed by the body, but not made by it in amounts sufficient to meet physiological needs and must therefore be obtained through diet. They include the omega-3 and omega-6 fatty acids.
References
Burr, G.O., Burr, M.M. and Miller, E. (1930). "On the nature and role of the fatty acids essential in nutrition". J. Biol. Chem. 86 (587). Retrieved on 2007-01-17.