A polyunsaturated fatty acid whose carbon chain has its first double valence bond three carbons from the beginning WordNet 3.0 & Open English WordNet

Omega−3 fatty acids, also called Omega−3 oils, ω−3 fatty acids or n−3 fatty acids, are polyunsaturated fatty acids characterized by the presence of a double bond, three atoms away from the terminal methyl group in their chemical structure. Wikipedia

Fatty acids with a double bond at the third carbon atom from the end of the carbon chain Wikidata

Any polyunsaturated fatty acid having a double bond between the third and fourth carbon atoms from the end of the molecule farthest from the carboxylic acid; they are found in green vegetables and in the oils of fish such as salmon and mackerel; they are essential fatty acids, and seem to be beneficial in reducing the risk of heart disease. Wiktionary

Any polyunsaturated fatty acid having a double bond. Wiktionary (translation)