Original: In chemistry, especially biochemistry, a fatty acid is a carboxylic acid with a long unbranched aliphatic tail (chain), which is either saturated or unsaturated. Most naturally occurring fatty acids have a chain of an even number of carbon atoms, from 4 to 28.
Simplified: Natural fatty acids commonly have a chain of four to 28 carbon atoma (usually unbranched and even numbered), which may be saturated or unsaturated.