A synthetic or semi-synthetic amorphous solid material, typically a polymer of high molecular weight made with monomers organic compounds that may be natural or synthetic in origin.
Plastics were a terrible pollutant in lo tech societies, because they are not biodegradable. They were used to a tremendous degree on Old Earth during the late Industrial Age and the Information Age for food-wrapping film (resulting in fouling of the foodstuff; strangely, this was not considered a problem), drink-containers, packing-materials, for furniture, the casings of electronic devices, and so on. While many of the original containers were recycled by bionano devices or with the assistance of gengineered organisms during the Interplanetary Age, and others, buried in landfills, have presumably been returned to their original fossil fuel state by GAIA, a number were still kept as mementos or useful storage bottles by Belters and other frontierspeople of the time. These eventually ended up in historical museums or private collections. Today an original unreplicated Pepsi, with its characteristic age-induced blackening (historical records indicate the original containers were partially transparent), can be worth anywhere up to several million credits, depending on its condition. There are many ruthless entrepreneurs and con-beings pushing cheap nanofabricated atomically precise replicas on the gullible, and before buying one is strongly advised to check the merchandise for the distinct atomic resonance signatures that indicate it is actually derived from pre-interplanetary age Old Earth.
Plastics of one sort or another are still widely used, though of course they are rarely derived from planetary hydrocarbon deposits and are fully integrated with biological, bionano, or other recycling systems.