The condition of a yarn or similar structure when the component elements have a helical disposition such as results, for instance, from relative rotation of the yarn ends. For all practical purposes twist is measured in turns, but for purely theoretical work its measurement in radians (the SI unit) often leads to much simpler mathematical expressions.
Textile Resource (http://www.textile.org.uk)
In spinning, this refers to the turns inserted into a yarn to bind its fibers together and thereby add more strength. It is usually indicated as turns per inch or tpi Greater twist would be caused by additional revolutions of the flyer on a set amount of fiber.
Textile Links (http://www.textilelinks.com)