Why are some fatty acids called 'essential fatty acids'?
Your body uses both essential and nonessential fatty acids.
Essential fatty acid (EFA) - this is a polyunsaturated fatty acid which can be obtained ONLY through the diet because your body can't make them. So, it is ESSENTIAL that you get them from your diet. There are two: Alpha-Linolenic-Acid (part of the family of Omega-3 fatty acids) and Linoleic-Acid (part of the family of Omega-6). - There are other types of Omega-3 and Omega-6 fatty acids which are nonessential.
Nonessential fatty acid - these can be made by the human body, so you don't need to obtain them from diet alone.
Essential fatty acids are fatty acids that cannot be constructed within an organism from other components by any known chemical pathways, and therefore must be obtained from the diet. The term refers to fatty acids involved in biological processes, and not those which may just play a role as fuel. As many of the compounds created from essential fatty acids can be taken directly in the diet, it is possible that the amounts required in the diet (if any) are overestimated. Essential fatty acids are of two types i.e omega 3 and omega 6 fatty acids.
Fatty acids are straight chain hydrocarbons possessing a carboxyl(COOH) group at one end. The carbon next to the carboxylate is known as α, the next carbon β, and so forth. Since biological fatty acids can be of different lengths, the last position is labelled as a "ω", the last letter in the Greek alphabet.