A gear is a rotating circular machine part that has cut teeth or a case. This case of cogwheel or gearwheel has an inserted tooth with mesh with another toothed part to transmit torque. It is also called a cog. Gears are devices that help in changing the speed, torque and direction of a power source. Gears were initially invented in the third century BC by great Archimedes, with widespread use in the Roman world.