Polyester - A general term encompassing all polymers in which the main polymer backbones are formed by the esterification condensation of polyfunctional alcohols and acids. There are two classes of polyesters, thermoplastics such as polybutylene terephthalate (PBT) and polyethylene terephthalate (PET), and thermosets. Thermoplastic polyesters are one of the fastest growing of the engineering thermoplastics. The reason for this is because polyesters fit into applications that are not readily filled by other engineering plastics, but they are also able to readily compete in applications where other engineering plastics are used as well.