"Polyester yarn" refers to yarn made from polyester-a type of fiber produced by spinning polymers. Currently, the term generally refers to fibers made from polyethylene terephthalate (commonly abbreviated as PET). In my country, this material is known as 'dilian' (polyester).
The term "yarn" (often used as a catch-all for both 'sha' and 'xian' in Chinese) actually encompasses two distinct categories. In the study of apparel materials, they are defined as follows: "*Sha*" (yarn/spun yarn) is a slender strand formed by arranging numerous staple fibers or continuous filaments in a roughly parallel alignment and twisting them along their axis to achieve specific strength and linear density; "Xian" (thread/ply yarn), on the other hand, is a plied strand created by twisting together two or more single yarns. In short, 'sha' consists of fine strands spun from materials such as cotton, wool, linen, or chemical fibers (typically serving as raw material for weaving), while *xian* is a multi-ply strand formed by twisting together these finer strands.