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The term vacuum has two different meanings. In its strictest sense, a vacuum is a region of space completely lacking any form of matter. The term represents absolute emptiness. One problem with this definition is that it describes an ideal condition that cannot exist in the real world. No one has ever discovered a way to make a perfect vacuum of this kind.

For that reason, the term vacuum also is used to describe regions of space from which the greatest possible amount of matter has been removed. In most cases, a vacuum is a container from which all gases have been removed as completely as possible.

Actually, the closest thing to a perfect vacuum is outer space. Astronomers believe that the space between stars consists in some cases of no more than a single atom or molecule per cubic kilometer. No vacuum produced on Earth comes even close to this condition.

The usual procedure for making a vacuum is with a vacuum...