black hole

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A massive star in the last phase of its evolution, in which the star collapses, creating a volume of space-time with a gravitational field so intense that its escape velocity equals or exceeds that of light.
• noun A great void; an abyss.

from The Century Dictionary.

• noun A dungeon or dark cell in a prison; a place of confinement for soldiers; any dismal place for confinement by way of punishment.

from the GNU version of the Collaborative International Dictionary of English.

• A dungeon or dark cell in a prison; a military lock-up or guardroom; -- now commonly with allusion to the cell (the Black Hole) in a fort at Calcutta (called the Black Hole of Calcutta), into which 146 English prisoners were thrust by the nabob Suraja Dowla on the night of June 20, 1765, and in which 123 of the prisoners died before morning from lack of air.
• (Physics, Astron.) An astronomical object whose mass is so condensed that the gravitational force does not allow anything, even light, to escape from its outer limit (the event horizon). The existence of such objects was first proposed from theoretical considerations. Because light cannot escape from such objects, they have not yet been detected with certainty (1998), but several "candidates" have been observed whose properties strongly suggest that they are black holes. Some theorists suggest that the centers of many galaxies may have large black holes at their cores. See also escape velocity.
• Fig., Jocose a place into which things may enter, but can never emerge.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A gravitationally domineering celestial body with an event horizon from which even light cannot escape; the most dense material in the universe, condensed into a singularity, usually formed by a collapsing massive star.
• noun A sphere of influence into which or from which communication or similar activity is precluded.
• noun An entity which consumes time or resources without demonstrable utility.
• noun A dungeon or dark cell in a prison; a military lock-up or guardroom.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a region of space resulting from the collapse of a star; extremely high gravitational field