An ergosphere is the region outside a rotating black hole horizon in which the stationary time-translation Killing vector is spacelike. Observers cannot remain stationary relative to infinity there. The Kerr ergoregion is its Kerr-metric instance.
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The term "ergosphere" refers to a region around a rotating black hole. In general relativity, the ergosphere is the area outside the event horizon where objects cannot remain in a fixed position due to the intense effects of the black hole's rotation. Within this region, the spacetime is dragged around in the direction of the black hole’s spin, a phenomenon known as "frame dragging.