A screw is a rod or tube with a helical groove on its surface. Its main use is as a fastener to hold two objects together, and as a simple machine for converting torque into linear force. A screw can also be defined as an inclined plane that wraps around a rod.
Most screws are tightened by turning them clockwise, which is called right-hand thread. Left-hand threaded screws are used in certain cases, for example when the screw is about to act counterclockwise. The left pedal of the bike has a left thread.