A swivel is a device that allows something to turn freely. Desk chairs are often designed so the seat can swivel atop the base. In the hospital, the TV is usually mounted on a swivel so that patients can adjust it.
Swivel comes from a middle English word, swive, meaning to sweep. It’s often used to describe gun mountings, like on a tank or a boat where a gun is fastened down but can still be swung around in any direction. But people can also swivel. If you were in the middle of robbing a bank and you heard a noise behind you, you’d probably swivel around quickly to see what it was.