A skein is a loosely wrapped, oblong coil of worsted yarn or floss. Yarn is wound on a reel into a skein, from which the material is easily pulled and rewound into a ball. Skeins provide easy, relatively tangle-free access to and use of the yarn for crocheting, knitting and tatting. Most crafters prefer to wind the skein into a large ball of yarn, to avoid knots and snarls that might occur when yarn is pulled too quickly from the skein or a defect in the winding process binds the yarn inside the skein.

For centuries before the mass production of textiles, women wove their homespun yarns into skeins manually, using a homemade wooden swift, a niddy-noddy or an extra pair of hands. Today, manufactured yarn is spun into skeins using a machine called a reel or weasel. Yarn skeins might be dyed before or after the spinning process. Skein dyeing is an ancient practice; skein dye pots have been discovered in the ruins of ancient Pompeii and South America.

In Europe, Canada and the United States, skeins of yarn are usually sold in units by weight and not lengths. Common skein weights are 3-ounce, 6-ounce and 8-ounce skeins and 25-grams, 50-grams and 100-grams skeins, depending on which system of measurement is used. There have been efforts to standardize skein weights with a universal numbering system. Skein weight of different types of yarn can vary greatly because of the bulkiness of the yarn, so a thick wool skein of 100 grams will be much smaller than a 100-gram skein of nylon yarn.

A skein of yarn is different than a hank of yarn. A hank is a very loose coil of yarn twisted into a large ring. The ring is twisted on itself, and the hank is packaged for sale. Handmade yarns, such as alpaca yarn or worsted wool, are frequently twisted into hanks.