This vacuum means "absence of matter" or "an empty area or space"; for the cleaning appliance, see vacuum cleaner. For the vacuum used in defining the MKS system of units, see free space. A 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," but in reality, no volume of space can ever be perfectly empty. A perfect vacuum with a gaseous pressure of absolute zero is a philosophical concept that is never observed in practice. Physicists often discuss ideal test results that would occur in a perfect vacuum, which they simply call "vacuum" or "free space" in this context, 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 otherwi

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