Plastics are a wide range of synthetic or semi-synthetic materials that use polymers as a main ingredient. - Wikipedia

: a plastic substance specifically : any of numerous organic synthetic or processed materials that are mostly thermoplastic or thermosetting polymers of high molecular weight and that can be made into objects, films, or filaments -Merriam Webster

plastic, polymeric material that has the capability of being molded or shaped, usually by the application of heat and pressure. This property of plasticity, often found in combination with other special properties such as low density, low electrical conductivity, transparency, and toughness, allows plastics to be made into a great variety of products. These include tough and lightweight beverage bottles made of polyethylene terephthalate (PET), flexible garden hoses made of polyvinyl chloride (PVC), insulating food containers made of foamed polystyrene, and shatterproof windows made of polymethyl methacrylate. -Brittanica

Composites like CF and fiberglass use those materials as a stress distributing component and a plastic component as the rigid layer.

Historically fiberglass composites used things like polyester resin - a polymer derived from oil - as the rigid component.

If that’s not a plastic, what is it then?

Metal? Wood? Glass?