Essential fatty acids, or EFAs, are fatty acids that humans and other animals must ingest because the body requires them for good health but cannot synthesize them.The term "essential fatty acid" refers to fatty acids required for biological processes but does not include the fats that only act as fuel. Essential fatty acids should not be confused with essential oils, which are "essential" in the sense of being a concentrated essence. Only two fatty acids are known to be essential for humans: alpha-linolenic acid (an omega-3 fatty acid) and linoleic acid (an omega-6 fatty acid). Some other fatty acids are sometimes classified as "conditionally essential", meaning that they can become essential under some developmental or disease conditions; examples include docosahexaenoic acid (an omega-3 fatty acid) and gamma-linolenic acid (an omega-6 fatty acid). When the two EFAs were discovered in 1923, they were designated "vitamin F", but in 1929, research on rats showed that the two EFAs are better classified as fats rather than vitamins. Functions
The biological effects of the ω-3 and ω-6 fatty acids are mediated by their mutual interactions, see Essential fatty acid interactions for detail.In the body, essential fatty acids serve multiple functions. In each of these, the balance between dietary ω-3 and ω-6 strongly affects function.
In free fatty acid form, the chemical structure of stearidonic acid is:

Examples
Polyunsaturated fatty acids with 16- and 18-carbon chains are sometimes classified as short chain polyunsaturated fatty acids (SC-PUFA), as opposed to long-chain polyunsaturated fatty acids (LC-PUFA), which have more than 18 carbon atoms.Both the essential fatty acids are SC-PUFA with an 18-carbon chain:

ω-3 fatty acid:
α-linolenic acid or ALA (18:3n-3)
ω-6 fatty acid:
linoleic acid or LA (18:2n-6)These two fatty acids cannot be synthesized by humans because humans lack the desaturase enzymes required for their production. They form the starting point for the creation of more desaturated fatty acids, most of which also have a longer carbon chain:

ω-3 fatty acids:
eicosapentaenoic acid or EPA (20:5n-3)
docosahexaenoic acid or DHA (22:6n-3)
ω-6 fatty acids:
gamma-linolenic acid or GLA (18:3n-6)
dihomo-gamma-linolenic acid or DGLA (20:3n-6)
arachidonic acid or AA (20:4n-6)Except for GLA, which has a short 18-carbon chain, these fatty acids have more than 18 carbon atoms and are typically classified as LC-PUFA.ω-9 fatty acids are not essential in humans because they can be synthesized from carbohydrates or other fatty acids. Essentiality in human diet
Mammals lack the ability to introduce double bonds in fatty acids beyond carbon 9 and 10, hence the omega-6 linoleic acid (18:2n-6; LA) and the omega-3 Alpha-linolenic acid (18:3n-3; ALA) are essential for humans in the diet. However, humans can convert both LA and ALA to fatty acids with longer carbon chains and a larger number of double bonds, by alternative desaturation and chain elongation. In humans, arachidonic acid (20:4n-6; AA) can be synthesized from LA.