Q&A: Black Holes
What are "ergosphere" and "stationary limit", and what is an "event horizon"?
A "stationary limit" is a surface around the outside of a rotating black
hole. Any object which passes through this surface is always in motion -
it's pulled by the distortion of space caused by the rotating black
hole. The "event horizon" is a surface that exists closer to the black
hole. Any object which passes through this surface can never be seen
again. Nothing, not even light, can escape from inside the event horizon
(this is how the event horizon is defined and that's why they're called
black holes). An "ergosphere" is the region in between the event horizon
and the stationary limit.
More information on Black Holes.