Vacuum

in theory, a space in which there is no matter. However, a perfect vacuum is unobtainable and the term describes a gas at a very low pressure.

Taken from Dictionary of Science

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In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". Even putting aside the complexities of the quantum vacuum, the classical notion of a perfect vacuum with gaseous pressure of exactly zero is only a philosophical concept and never is observed in practice. Physicists often discuss ideal test results that would occur in a perfect vacuum, which they simply call "vacuum" or "free space", and use the term partial vacuum to refer to real vacuum. The Latin term in vacuo is also used to describe an object as being in what would otherwise be a vacuum.

The quality of a vacuum refers to how closely it approaches a perfect vacuum. Residual gas pressure is a primary indicator of quality, and is most commonly measured in units called torr, even in metric contexts. Lower pressures indicate higher quality, although other variables must also be taken into account. See ultra-high vacuum. Quantum theory sets limits for the best possible quality of vacuum, predicting that no volume of space can be perfectly empty. See QCD vacuum, for example. Outer space and interstellar space are naturally occurring high quality vacuums, mostly of much higher quality than can be created artificially with current technology. Low quality artificial vacuums have been used for suction for many years.

Vacuum has been a frequent topic of philosophical debate since Ancient Greek times, but was not studied empirically until the 17th century. Evangelista Torricelli produced the first laboratory vacuum in 1643, and other experimental techniques were developed as a result of his theories of atmospheric pressure. A torricellian vacuum is created by filling a tall glass container closed at one end with mercury and then inverting the container into a bowl to contain the mercury.

Vacuum became a valuable industrial tool in the 20th century with the introduction of incandescent light bulbs and vacuum tubes, and a wide array of vacuum technology has since become available. The recent development of human spaceflight has raised interest in the impact of vacuum on human health, and on life forms in general.

Taken from Wikipedia