Is polyester the scientific name for plastic?

Not always, although the term is widely used for plastics such as polystyrene and polyethylene, and for artificial fibers such as Dacron, which like plastic bottles is a form of polyethylene terephthalate (PET). There are some natural polyesters.

A "polyester" is any natural or synthetic polymer (repeated chain molecule) containing esters. But the term is usually applied to two groups of synthesized molecules: thermoplastics and thermosets.