PETG, formally known as Polyethylene terephthalate glycol-modified, is based on PET (a very common plastic used in Thermoforming processes) but adapted to be clearer, less brittle, and easier to use.

Because PET is an excellent water and moisture barrier material, plastic bottles made from PET are widely used for soft drinks (see carbonation). For certain specialty bottles, such as those designated for beer containment, PET sandwiches an additional polyvinyl alcohol (PVOH) layer to further reduce its oxygen permeability.

Sources: https://en.wikipedia.org/wiki/Polyethylene_terephthalate
https://all3dp.com/petg-filament-3d-printing/