Any unsaturated hydrocarbon containing one or more pairs of carbon atoms linked by a double bond (see covalent 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.

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Alpha-olefins (or α-olefins) are a family of organic compounds which are olefins or alkenes with a chemical formula C_xH_2x, distinguished by having a double bond at the primary or alpha (α) position. See the illustration below. This location of a double bond enhances the reactivity of the compound and makes it useful for a number of applications.

There are two types of alpha-olefins, branched and linear (or normal). The chemical properties of branched alpha-olefins with a branch at either the second (vinylidene) or the third carbon number are significantly different from the properties of linear alpha-olefins and those with branches on the fourth carbon number and further from the start of the chain.