Polyethylene terephthalate, polyethylene terephthalate, polyethylene terephthalate or polyethylene terephthalate (better known by its acronym in English PET, polyethylene terephthalate) is a type of plastic widely used in beverage containers and textiles. Some companies manufacture PET and other polyesters under different brand names that have come into common use, for example, in the United States and the United Kingdom they use the names Mylar and Melinex.

Chemically, PET is a polymer that is obtained through a polycondensation reaction between terephthalic acid and ethylene glycol. It belongs to the group of polyesters called synthetic materials.

It is a linear thermoplastic polymer, with a high degree of crystallinity. Like all thermoplastics, it can be processed by extrusion, injection, injection and blow molding, preform blow molding and thermoforming. To prevent excessive growth of spherulites and crystal lamellae, this material must be rapidly cooled, thereby achieving greater transparency. The reason for their transparency on rapid cooling is that the crystals do not grow fully and their size does not interfere (“scattering”) with the path of the wavelength of visible light, according to quantum theory.