(type of region of space around rotating black hole)
An ergosphere is a defined region of space surrounding a rotating
black hole.
According to general relativity (GR), the space surrounding such a black hole
rotates along with it, space's rotation being quickest adjacent to
the event horizon and falling off with distance from the black
hole, this rotation of the surrounding space termed frame dragging.
The ergosphere is defined to be the set of locations where this
dragged space is moving the speed of light or faster in relation
to distant space, e.g., in relation to us.
The impossibility of moving beyond the speed of light applies to
the relative speed of objects both within the same relatively-flat
space, but does not apply to objects distant from each other between
which space is being so-deformed.