Polyethylene is a polymer consisting of long chains of the monomer ethylene (IUPAC name ethene). The recommended scientific name polyethene is systematically derived from the scientific name of the monomer ^[1][2]. In certain circumstances it is useful to use a structure-based nomenclature; in such cases IUPAC recommends poly(methylene) ^[2] (poly(methanediyl) is an non-preferred alternative ^[3][4]). The difference in names between the two systems is due to the opening up of the monomer's double bond upon polymerisation.

In the polymer industry the name is sometimes shortened to PE in a manner similar to that by which other polymers like polypropylene and polystyrene are shortened to PP and PS respectively. In the United Kingdom the polymer is commonly called polythene, although this is not recognized scientifically.

The ethene molecule (known almost universally by its common name ethylene) C₂H₄ is CH₂=CH₂, Two CH₂ groups connected by a double bond, thus:

Polyethylene contains the chemical elements carbon and hydrogen.

Polyethylene is created through polymerization of ethene. It can be produced through radical polymerization, anionic addition polymerization, ion coordination polymerization or cationic addition polymerization. This is because ethene does not have any substituent groups that influence the stability of the propagation head of the polymer. Each of these methods results in a different type of polyethylene.