In science, a vacuum is a space devoid of matter or air. It is a region with an extremely low pressure, and partial vacuums are more common than perfect vacuums, which are theoretical spaces completely devoid of matter. A vacuum can be created by removing air from a space using a vacuum pump or by reducing the pressure using a fast flow of fluid, as in Bernoulli’s principle. The word "vacuum" comes from the Latin adjective vacuus for "vacant" or "void".

Some examples of a vacuum include the inside of an incandescent lightbulb, space, the thin atmospheres of the Moon, Mercury, and Mars, suction from a vacuum cleaner, the insulating area between the glass walls of a thermos, and the Earth’s thermosphere.

Vacuum technology is usually associated with three types of flow: viscous or continuum flow, molecular flow, and a transitional range between these two known as Knudsen flow. Vacuum became a valuable industrial tool in the 20th century with the introduction of incandescent light bulbs and vacuum tubes, and a wide array of vacuum technologies has since become available.

In summary, a vacuum is a space devoid of matter or air, and it can be created by removing air from a space or reducing the pressure using a fast flow of fluid. Partial vacuums are more common than perfect vacuums, and examples of a vacuum include the inside of an incandescent lightbulb, space, and suction from a vacuum cleaner. Vacuum technology is associated with three types of flow: viscous or continuum flow, molecular flow, and a transitional range between these two known as Knudsen flow.