Any of a class of lipids consisting of various saturated or unsaturated, monobasic organic acids, generally R.COOH, usually in a straight chain. Chain length ranges from one hydrogen atom (methanoic, or formic, acid HCOOH) to nearly 30 carbon atoms.

Long chain fatty acids (more than 8-10 carbon atoms) most commonly occurs 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 hexadecanoic) acid and stearic or unsaturated, with one double bond (e.g. oleic cis-octodec-9-enoic-acid) or two or more double bonds, in which case they are called polyunsaturated fatty acids(e.g., linoleic acid and linolenic acid)

Short chain acids are pungent liquids, soluble in water.

As chain length increases, melting points are raised and water-solubility decreases.