The Kerr metric describes the spacetime around a rotating, uncharged black hole.
Roy Kerr published the solution in 1963 as an exact solution to Einstein’s field equations. It extends the simpler Schwarzschild solution, which describes a nonrotating, uncharged spherical mass. Rotation changes the geometry significantly: outside the event horizon, frame dragging forces spacetime itself to rotate relative to distant observers.
A rotating black hole has an event horizon and an ergosphere. Inside the ergosphere, no observer can remain stationary relative to distant space. In principle, rotational energy can be extracted from this region through the Penrose process or related electromagnetic mechanisms.
The Kerr solution assumes an idealized isolated black hole with no electric charge. Astrophysical black holes are expected to be nearly neutral because surrounding plasma would quickly cancel large charge, while their spins can be substantial. Measuring spin helps astronomers test models of accretion and black-hole growth.