Fatty acid

A fatty acid is a carboxylic acid (-C(=O)OH), with a long unbranched hydrocarbon tail. It is studied in organic chemistry and biochemistry. 
Fatty acids are important sources of body fuel. When metabolized, they yield large quantities of ATP. Many cell types can use either glucose or fatty acids for this purpose. In particular, heart and skeletal muscle prefer fatty acids. 
Fatty acids may be either saturated or unsaturated. Unsaturated compounds have reactive double bonds; saturated ones do not.
Fatty acids are aliphatic monocarboxylic acids derived from, or contained in, an animal or vegetable fat, oil, or wax. Natural fatty acids commonly have a chain of four to 28 carbon atoms (usually unbranched and even numbered), which may be saturated or unsaturated. This would include acetic acid, although this is not usually considered a fatty acid (not a lipid). 
The blend of fatty acids in mammalian skin, together with lactic acid and pyruvic acid, are distinctive. They allow animals with a keen sense of smell to identify individuals.