What is a ball cock?

It is a water valve that is operated by a ball on the end of a lever.

The valve that shuts off the water in a toilet cistern is known as the ball-cock. It is operated by a hollow ball attached to an arm, once the ball floats to a certain level, the valve shuts off the water. This action prevents water flowing out of the over-flow pipe to the outside. The ball and valve is known as the ball-cock.