Any unsaturated hydrocarbon containing one or more pairs of carbon atoms linked by a double bond (seecovalent bond, saturation). Olefins may be classified by whether the double bond is in a ring (cyclic) or a chain (acyclic, or aliphatic) or by the number of double bonds (monoolefin, diolefin, etc.). Rare in nature, olefins are obtained by the cracking of petroleum fractions at high temperatures. The simplest ones (ethylene, propylene, butylene, butadiene, and isoprene) are the basis of the petrochemicals industry. They react by adding other chemical agents at the double bond to form derivatives or polymers.