Spandex (also referred to as Lycra or elastane) is a synthetic fiber prized for its elasticity & durability.
Spandex is most commonly used in conjunction with other fabrics to add stretch to jeans, tops, dresses etcetera. Even the stretchiest bathing suits or leotards generally comprise of less than 40 percent spandex mixed with cotton or polyester.
When spandex is used to add stretch to a fabric it is rarely visible as it accounts for such a small portion of the blend, meaning that the fabric maintains its original appearance. For example, spandex is often added to denim to allow a little stretch in jeans, however the final fabric still looks like denim.