Synthetic Fibers
Fibers manufactured from chemical processes rather than grown or harvested — polyester, nylon, acrylic, and spandex among the most common in garment sewing.
Where cotton, silk, wool, and linen come from plants and animals, synthetic fibers are manufactured — most of them, including polyester and nylon, spun from petroleum-derived polymers, extruded through tiny holes into continuous filaments. Spandex (branded as Lycra) is the synthetic that made stretch fabric possible at scale, usually blended in small percentages with another fiber rather than used alone.
My honest take, having worked with plenty of both: synthetics get an unfairly bad reputation among sewists that's mostly a holdover from cheap polyester's reputation in the 70s and 80s. A good quality synthetic — a proper polyester crepe, a technical performance knit — can drape, breathe, and behave better than a low-quality natural fiber. Where synthetics genuinely differ in handling: they don't take a press the same way natural fibers do (many will scorch or melt under too hot an iron rather than just singe), they don't absorb moisture the way cotton or wool do, and pure polyester frays less but also doesn't ease or ravel the way woven cotton does, which changes how forgiving certain techniques are.
Fiber content actually matters for technique choices, not just fabric shopping preference — always check the burn test or the bolt label before you commit an iron setting or a seam finish to a fabric you don't recognize.