Slewing is a scientific term for when a rotating object moves on its axis. It's used both as a mathematical description for a type of movement and a term for parts in machinery created to facilitate that kind of movement. The term also has several everyday uses in popular culture.
One type of slewing can be illustrated with the movement of a spinning top. If a top has been given a firm spin, it will remain level with its axis. As the spin progresses, the axis of the top will wobble while it spins. This is the "slewing" movement.
Another kind of slewing is simply a range of vertical and horizontal movement. The Large Array, a series of radio satellite dishes, are programmed to follow the movement of the sun. Their dishes rotate in a side-to-side movement every day, turning on their axes. Similarly, a spaceship is said to perform a slewing motion when it changes its altitude.
The term slewing can also be used in a less scientific way. It can mean that something has made an error in a set path or is making a sudden, out-of-control movement. A car skidding in slush in such a way that it changes its direction may be said to have "slewed around." "Slewing" is also a Canadian term for the act of getting drunk.
The term is also used to describe several kinds of mechanical parts used to create a rotating joint, such as ball bearings. They need to be strong enough to bear a load coming from many different directions. These kinds of bearings can come in both cylindrical and ball styles.
A slewing ring is a double-sided metal ring. The two rings rotate around each other on a series of cogs or ball bearings. Often, two rows of bearings will be used inside a pair of rings in order to make a part with a rotating axis.
These parts are used in creating cranes and other large machines. They are often manufactured for use in mechanical objects such as windmills and Ferris wheels. Other times, slewing rings can be used in building things on a smaller scale. For example, robot arms, whose joints require a great range of motion, often make use of slewing rings and bearings. These parts are good for robots created for tasks which require fine motor control and can be used to create parts similar to wrist joints.