Omega-3 fatty acids (also called ω−3 fatty acids or n−3 fatty acids) are fats commonly found in marine and plant oils. They are polyunsaturated fatty acids with a double bond (C=C) at the third carbon atom from the end of the carbon chain. The fatty acids have two ends; the acid (-COOH) end, which is considered the beginning of the chain, thus "alpha", and the methyl (CH3) end, which is considered the "tail" of the chain, thus "omega". The nomenclature of the fatty acid is taken from the location of the first double bond, counted from the methyl end, that is, the omega (ω-) or the n- end. Long-chain omega−3 fatty acids seem to have anti-inflammatory activity.
Omega-3 fatty acids are considered essential fatty acids, meaning that they cannot be synthesized by the human body but are vital for normal metabolism. Though mammals cannot synthesize omega−3 fatty acids, they have a limited ability to form the long-chain omega−3 fatty acids including eicosapentaenoic acid (EPA, 20 carbons and 5 double bonds), docosahexaenoic acid (DHA, 22 carbons and 6 double bonds), and α-linolenic acid (ALA, 18 carbons and 3 double bonds).