In the simplest sense, empty space. However, since a vacuum, either natural or artificial, is never completely empty, the term needs a modifier. Thus scientists speak of a hard vacuum (one that approximates the vacuum of outer space), quantum vacuum, and so forth. The best man-made vacuums contain less than 100,000 moecules per cubic centimeter.
Evangelista Torricelli is credited with developing the first artificial vacuum in the mercury barometer. In 1650 the German physicist Otto von Guericke invented the first vacuum pump.
See also Casimir effect, vacuum energy drive, and zero point energy.
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