black hole

	1.	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. A discipline of unlimited autocracy, upheld by rods, and ferules, and the black hole. - H. Spencer.
	2.	(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.
	3.	a place into which things may enter, but can never emerge.

Noun

1.

black hole - a region of space resulting from the collapse of a star; extremely high gravitational field

1.		black hole - An expression which depends on its own value or a technique to detect such expressions. In graph reduction, when the reduction of an expression is begun, the root of the expression can be overwritten with a black hole. If the expression depends on its own value, e.g. x = x + 1 then it will try to evaluate the black hole which will usually print an error message and abort the program. A secondary effect is that, once the root of the expression has been black-holed, parts of the expression which are no longer required may be freed for garbage collection. Without black holes the usual result of attempting to evaluate an expression which depends on itself would be a stack overflow. If the expression is evaluated successfully then the black hole will be updated with the value. Expressions such as ones = 1 : ones are not black holes because the list constructor, : is lazy so the reference to ones is not evaluated when evaluating ones to WHNF.
2.		black hole - Where an electronic mail message or news aritcle has gone if it disappears mysteriously between its origin and destination sites without returning a bounce message. Compare bit bucket.

part, region