A steady state solution of Einstein's equations representing a rotating black hole discovered in 1963 by the mathematician Roy Kerr. Like all black holes, this one is defined by the existence of an event horizon, but it also has a region of finite size extending all around it called the ergosphere. Any object that initially falls radially that penetrates the ergosphere will be subjected to a force making it rotate around the black hole.
As the astrophysicist and Nobel prize winner Chandrasekhar pointed out, black holes are the simplest objects in the universe because they are completely defined by only 4 parameters: the mass M, the angular velocity J and the electrical and magnetic charges Q and P. In the case of Kerr there are only M and J, and in general relativity it has been shown that a black hole is necessarily described by a single solution depending on these parameters, the Kerr-Newman solution.
Kerr black holes are considered to be the largest energy reservoirs in the universe. They are used to explain the extraordinary energy source of active galaxy cores, quasars, using a mechanism proposed by Blandford and Znajeck.