ETYM Old Eng. scrue, Old Fren. escroue, escroe, female screw, French écrou, Latin scrobis a ditch, trench, in Late Lat., the hole made by swine in rooting; cf. Dutch schroef a screw, German schraube, Icel. skrűfa.
1. A fastener with a tapered threaded shank and a slotted head.
2. A simple machine of the inclined-plane type consisting of a spirally threaded cylindrical rod that engages with a similarly threaded hole.
3. A device with several angled blades that rotates to push against water or air; SYN. screw propeller.
In construction, cylindrical or tapering piece of metal or plastic (or formerly wood) with a helical groove cut into it. Each turn of a screw moves it forward or backward by a distance equal to the pitch (the spacing between neighboring threads).
Its mechanical advantage equals 2 r/P, where P is the pitch and r is the radius of the thread. Thus the mechanical advantage of a tapering wood screw, for example, increases as it is rotated into the wood.
The thread is comparable to an inclined plane (wedge) wrapped around a cylinder or cone.