Definition of plastic clarifies material scope and manufacturing characteristics for legal and regulatory classification purposes. Definition of 'plastic' covers polymeric substances, natural or synthetic, that can be moulded and are distinct from elastomers and fibres; engineering definitions add that plastics are resins compounded with fillers, plasticizers, lubricants, dyes and other additives and thereafter fabricated. Descriptive sources stress the capacity to be shaped by heat or pressure and to retain a solid form, and identify high molecular weight materials produced by polymerization or chemical treatment of natural substances.