Worsted is a high-quality type of wool yarn, the fabric made from this yarn, and a yarn weight category. The name derives from Worstead, a village in the English county of Norfolk. That village, together with North Walsham and Aylsham, formed a manufacturing centre for yarn and cloth in the 12th century, when pasture enclosure and liming rendered the East Anglian soil too rich for the older agrarian sheep breeds. In the same period, many weavers from the County of Flanders moved to Norfolk. "Worsted" yarns/fabrics are distinct from woollens : the former is considered stronger, finer, smoother, and harder than the latter.

Yarn is a long continuous length of interlocked fibres, used in sewing, crocheting, knitting, weaving, embroidery, ropemaking, and the production of textiles. Thread is a type of yarn intended for sewing by hand or machine. Modern manufactured sewing threads may be finished with wax or other lubricants to withstand the stresses involved in sewing. Embroidery threads are yarns specifically designed for needlework. Yarn can be made of a number of natural or synthetic materials, and comes in a variety of colors and thicknesses. Although yarn may be dyed different colours, most yarns are solid coloured with a uniform hue.