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How do omega-3 and omega-6 fatty acids get their names?

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A composite diagram shows the structural formulas, space-filling models, and simplified depictions of lipid molecules including stearic acid and phosphatidylcholine, and how these assemble into a micelle and lipid bilayer, with labels identifying hydrophilic heads and hydrophobic tails. — Lipid Structure and properties of two representative lipids. Both stearic acid (a fatty acid) and phosphatidylcholine (a phospholipid) are composed of chemical groups that form polar “heads” and nonpolar “tails.” The polar heads are hydrophilic, or soluble in water, whereas the nonpolar tails are hydrophobic, or insoluble in water. Lipid molecules of this composition spontaneously form aggregate structures such as micelles and lipid bilayers, with their hydrophilic ends oriented toward the watery medium and their hydrophobic ends shielded from the water.

Omega-3 and omega-6 fatty acids are named based on the position of the first double bond in their hydrocarbon chain. This position is determined by counting the carbon atoms starting from the end of the molecule most distant from the carboxylic acid group (the "omega" end).1

Omega-3 fatty acids: The first double bond occurs between the third and fourth carbon atoms from the omega end.2
Omega-6 fatty acids: The first double bond occurs between the sixth and seventh carbon atoms from the omega end.1

These fatty acids are essential components of lipids and are critical for various cellular processes. Because many animals cannot synthesize the parent compounds—linoleic acid for omega-6 and alpha-linolenic acid for omega-3—they must be obtained through the diet, classifying them as essential fatty acids.2