Silk is a satiny smooth fiber produced by silkworms for their cocoons. The silk industry takes these cocoons—with the metamorphosizing caterpillar inside—and processes its fiber into silk threads and fabrics.
Silk was once used far more widely in clothing than it is today. For more than a century prior to World War II, silk stockings were ubiquitous among well-dressed women. But the invention of nylon in 1938 allowed for stockings that were shearer, stronger, and much cheaper than those made from silk.
Modern fibers like nylon, rayon, and polyesters have substantially eaten into silk’s share of the clothing market. Silk remains commonly used in men’s suits and ties. While most other apparel rarely contains silk, it shows up enough in fancier shirts, dresses, and sweaters frequently enough that you should always check the label.
Silk is easy to avoid simply by checking the garment label. It’s generally easy to find apparel woven from rayon or from synthetic fibers rather than from silk.