Vacuum

A region whose pressure is far below atmospheric pressure, usually containing some particles. Its degree of vacuum depends on how few particles remain.

Vacuum pump: A device that removes gas from a sealed volume to lower its pressure. Pumps create low pressure by transferring gas out of the region.
Vacuum tube: An electronic device that controls electrical current through electrodes in an evacuated enclosure. Removing gas prevents electrical conduction through the tube's interior.
Pressure: The force exerted per unit area by a fluid or gas on a surface. Vacuum is defined by pressure being much lower than the surrounding atmosphere.
Perfect vacuum: An idealized region containing no matter and having zero pressure. Practical vacuums contain particles, unlike this unattainable idealization.
Mean free path: The average distance a particle travels between collisions with other particles. As pressure falls, particles travel farther between collisions, changing gas flow.
Electron microscope: A microscope that uses electron beams to form images at very small scales. A vacuum lets electrons travel without substantial scattering by gas molecules.
Atmospheric pressure: The pressure exerted by the weight of the atmosphere above a surface. It supplies the reference level from which vacuum pressure is judged.
Outer space: The vast region beyond Earth's atmosphere, containing extremely sparse matter and radiation. It is a natural low-pressure environment, but not a perfect vacuum.
Outgassing: The release of gas previously absorbed, adsorbed, or trapped in a material. Chamber walls and components release gas that limits achievable pressure.
Vacuum insulation panel: A thermal insulation panel whose evacuated core suppresses gas conduction and convection. Reducing gas pressure inside the panel limits two routes of heat transfer.

Linked from 21 pages