What is a ring singularity?

In rotating 'Kerr-type' black holes, the singularity is not a mathematical point at the center the way it is for the non-rotating Schwarschild black holes.

Because of 'centrifugal' forces, rotating black hole singularities are distorted into a ring-like region in the plane of rotation of the black hole. This is unobservable and hidden inside the event horizon, and is only a prediction of what is called a 'vacuum solution' to Einstein's field equations for gravity.