Silk is a very smooth, light fabric that's usually made of fibers from silkworm cocoons. Your sister might decide to wear a white dress made of silk to her wedding.
Silk is often an expensive kind of fabric from which fancy curtains and upholstery are made, as well as clothing. Things made of silk are soft and satiny, and they usually have an almost glossy sheen. The word silk has an Old English root, sioloc, from Sēres, the Greek word for people from the region in Asia where silk was first made.