Ergosphere

The region around a spinning black hole where space itself is dragged round with it.

A rotating black hole drags the spacetime around it, and inside the ergosphere that dragging is so strong that nothing can remain still relative to the distant stars — everything is forced to orbit. It sits outside the event horizon, so escape is still possible, and Roger Penrose showed that an object entering it and splitting in two can leave with more energy than it arrived with, stolen from the black hole's rotation. The leading models of relativistic jets run on exactly that.