A fatty acid is a carboxylic acid with a long unbranched hydrocarbon tail, which is either saturated or unsaturated. Biological fatty acids usually contain an even number of carbon atoms. Most double bonds in biological fatty acids are cis in configuration.

Soaps and detergents are fatty acid salts. The carboxylate group is hydrophilic while the hydrocarbon chain is lipophilic. Placed in water, soaps form a colloidal dispersion of spherical aggregates called micelles.