black hole

A place of punitive confinement; a lockup or cell; a military guardroom.

(astronomy) 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.

(figuratively) A void into which things disappear, or from which nothing emerges; an impenetrable area or subject; an area impervious to communication.

(aviation) A dangerous optical illusion that can occur on a nighttime approach with dark, featureless terrain between the aircraft and a brightly-lit runway, where the aircraft appears to the pilots to be higher up than it actually is, potentially triggering a premature or overly-steep descent and a crash short of the runway.

(Internet, often attributive) A place where incoming traffic is silently discarded.

(programming) A bit bucket; a place of permanent oblivion for data.