Or synthetic, refers to fibres that came into being after World War 2 following the ensuing scarcity of natural fibres.
These are natural fibres that have been chemically produced or altered and that stand alone as viscose, spun viscose, bemberg, cupro, among others…
Or they may be mixed with natural fibres, especially cotton, to gain the benefit of the inherent properties of each.
** see wording used under synthetic and tie in**