Vacuum, a space in which there is no matter. In a strict sense, there is no such thing as a vacuum—even the virtually empty regions of the universe between the galaxies contain some matter. In common usage, the term "vacuum" is used to refer to a space from which a gas or a mixture of gases, such as air, has been partially removed so that it has a relatively low pressure. Scientists and engineers generally speak of such a partial vacuum as being a low vacuum when the pressure is slightly less than atmospheric pressure, and as a high vacuum when the pressure is much less. In the laboratory, the highest vacuum that has been achieved is one having a pressure one-billionth of a billionth the standard pressure of the atmosphere.

Low vacuums are used in the operation of such devices as medicine droppers, vacuum cleaners, and suction cups. High vacuums are used in electron tubes, vacuum bottles, and various kinds of scientific research equipment. They are also used in a number of industrial processes, including the refining of metals and the drying of materials. There are many types of vacuum pumps for removing a gas from a container to achieve a high vacuum. The type chosen depends on the proportion and kind of gas to be evacuated.