An organic compound consisting of a hydrocarbon chain and a terminal carboxyl group (see carboxylic acids). Chain length ranges from one hydrogen atom (methanoic, or formic, acid, HCOOH) to nearly 30 carbon atoms. Ethanoic (acetic), propanoic (propionic), and butanoic (butyric) acids are important in metabolism. Long-chain fatty acids (more than 8–10 carbon atoms) most commonly occur as constituents of certain lipids, notably glycerides, phospholipids, sterols, and waxes, in which they are esterified with alcohols. These long-chain fatty acids generally have an even number of carbon atoms; unbranched chains predominate over branched chains. They may be saturated (e.g. palmitic acid and stearic acid) or unsaturated, with one double bond (e.g. oleic acid) or two or more double bonds, in which case they are called polyunsaturated fatty acids (PUFAs; e.g. linoleic acid and linolenic acid). A system of symbols is used for unsaturated fatty acids to denote their chain length (i.e. number of carbon atoms) and the number and position of their double bonds. For example, linoleic acid has 18 carbons and two double bonds, the first of which starts on the sixth carbon from the methyl end; it is denoted as C18:2, n-6 (or omega-6). In contrast, α‎-linolenic acid, which has 18 carbon atoms and three double bonds, the first starting on the third carbon from the methyl end, is denoted as C18:3, n-3 (or omega-3). See also essential fatty acids; trans fatty acid.

The physical properties of fatty acids are determined by chain length, degree of unsaturation, and chain branching. Short-chain acids are pungent liquids, soluble in water. As chain length increases, melting points are raised and water-solubility decreases. Unsaturation and chain branching tend to lower melting points.