A black hole is a region of space with a gravitational field which is too dense for anything that has traveled beyond its event horizon to ever escape. A true black hole is inescapable for any being below lightspeed and cannot be destroyed, as anything that attempts to due so would become infinitely dense and turn into a black hole itself. A true black hole also cannot be survived, as the tidal forces of gravity become infinite near the center and destroy anything within it on a quantum level.
Several gravity wells in fiction have been incorrectly referred to as black holes despite displaying none of the above characteristics. Caution must be taken in determining if a fictional black hole is a true one.