olefin , also called alkene , compound made up of hydrogen and carbon that contains one or more pairs of carbon atoms linked by a double bond. Olefins  are examples of unsaturated hydrocarbons (compounds that contain only hydrogen and carbon and at least one double or triple bond). They are classified in either or both of the following ways: (1) as cyclic or acyclic (aliphatic) olefins, in which the double bond is located between carbon atoms forming part of a cyclic (closed-ring) or of an open-chain grouping, respectively, and (2) as monoolefins, diolefins, triolefins , etc., in which the number of double bonds per molecule is, respectively, one, two, three, or some other number.olefin-definition1