- A hypothetical space which contains no matter, and whose absolute pressure is defined as zero. free space approaches such a vacuum. Also called absolute vacuum, or perfect vacuum.
- A region of space, an environment, or a system whose pressure is below a given threshold, such as 10-12 torr.
- A region of space, an environment, or a system whose pressure is significantly below atmospheric pressure. A low vacuum has a pressure between 760 and 1 torr, a medium vacuum has a pressure between 1 and 10-3 torr, a high vacuum between 10-3 and 10-6 torr, a very high vacuum between 10-6 and 10-9 torr, an ultra-high vacuum between 10-9 and 10-12 torr, and an extremely-high vacuum has a pressure lower than 10-12 torr.
- A region of space, an environment, an enclosure, or a system in which the pressure of any gases, such as air, is sufficiently low so as to not affect any desired or monitored characteristics or parameters. For example, a vacuum tube is an electron tube which is evacuated to a degree that any residual gas present does not affect its electrical characteristics.
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