According to Wikipedia:
Plastic is material consisting of any of a wide range of synthetic or semi-synthetic organic compounds that are malleable and so can be molded into solid objects.
Plastics are typically organic polymers of high molecular mass and often contain other substances.
The word ‘Plastic’ refers to something which is flexible or malleable. Although most synthetic polymers are flexible, there are a many materials which also have these properties including lead, butter, clay, and beeswax.
The word ‘Plastics’ refers to polymeric materials… these can be synthetic or natural, thermoset or thermoplastic, and semi-crystalline or amorphous.