A natural fiber reeled from the cocoon of a worm is silk. More precisely, commercial silk is produced mainly by the larva of the domesticated mulberry silkworm, Bombyx mori, which spins a cocoon from one continuous protein filament called fibroin, coated with sericin.
Silk production, known as sericulture, began in ancient China. According to traditional Chinese accounts, Empress Leizu discovered silk around 2700 BCE after a cocoon fell into her tea. Whether legendary or not, China kept silk-making secret for centuries, and the material later traveled west along the Silk Road.
The cocoon is softened in hot water so the filament can be found and reeled off. Several filaments are combined to create a stronger thread for weaving. Not all silk comes from domesticated silkworms: wild silks, including tussar and eri, come from other moth species and often have shorter, less uniform fibers.
A common mix-up is calling silk a plant fiber like cotton or linen. Silk is an animal-derived protein fiber, valued for its sheen, lightness, strength, and ability to regulate warmth.