Essential fatty acid: An unsaturated fatty acid that is essential to human health, but cannot be manufactured in the body. Abbreviated EFA. There are three types of EFAs: arachnoidic acid, linoleic acid, and linolenic acid. When linoleic acid is obtained in the diet, it can be converted to both arachnoidic and linolenic acid. Linoleic acid is commonly found in cold-pressed oils, especially oils extracted from cold-water fish and certain seeds. Supplementation with EFAs appears to be useful as a treatment for certain neurological disorders. However, arachnoidic acid may lower the seizure threshold. For that reason, it is important to consult a physician before starting a program of EFA supplementation.