Polyester is a type of polymer that has the ester functional group in its main chain. There are many different polyesters. The term "polyester" as a specific material most often refers to polyethylene terephthalate (PET). Polyesters include chemicals found in nature, such as in plant cuticles. Synthetics made by step-growth polymerization such as polycarbonate and polybutyrate are also polyesters. Natural polyesters and a few synthetic ones are biodegradable, but most synthetic polyesters are not.