- The zone from which no radiation can normally escape, formed in the collapse of a star of just over two solar masses or more. The theoretical description of a black hole, where time slows to a halt and other bizarre physical effects occur, was drawn up in 1916, and since then further complications (like the addition of rotation and electrical charge) have been added, as well as observational knowledge of objects like the Crab Nebula which seem to contain black holes. The cores of galaxies such as our own seem to contain massive black holes of millions of solar masses, which can be detected by measuring the orbital motion of stars near the centre of the galaxy. Their velocities betray the presence of an anomalously large mass at the core of the galaxy.
- Within a communications network, a location where data enters, but never leaves. May be created, for instance, by noncompatible equipment.
- An object in space whose gravitation is so great that neither matter nor light can escape from it.
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