black hole

n. theoretical space object which has an extremely strong gravitational force; dungeon

A black hole is a region of space in which the gravitational field is so powerful that nothing can escape after having fallen past the event horizon. The name comes from the fact that even electromagnetic radiation (e.g. light) is unable to escape, rendering the interior invisible. However, black holes can be detected if they interact with matter outside the event horizon, for example by drawing in gas from an orbiting star. The gas spirals inward, heating up to very high temperatures and emitting large amounts of radiation in the process.
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Noun
1. a region of space resulting from the collapse of a star; extremely high gravitational field
(hypernym) region, part

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, into which 146 English prisoners were thrust by the nabob Suraja Dowla on the night of June 20, 17656, and in which 123 of the prisoners died before morning from lack of air.
  

1. 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. 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.
[Jargon File]