In physics, a vacuum is a space devoid of matter. It is a condition where the pressure is so low that any particles in the space do not affect any processes being carried on there. 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.

It is important to note that in practice, only partial vacuums are possible. Outer space can approach the requirements of a vacuum, but it is not a perfect vacuum. The definition of a vacuum is not precise but is commonly taken to mean pressures below, and often considerably below, atmospheric pressure.

In vacuum science, there are different pressure ranges that are sub-divided into five individual regimes: high vacuum, very high vacuum, ultra-high vacuum, extreme high vacuum, and outer space vacuum. The physical properties of gases change with pressure, and different vacuum gauges are required to accommodate these changes.

In classical electromagnetism, the vacuum of free space is a standard reference medium for electromagnetic effects. In quantum mechanics and quantum field theory, the vacuum is defined as the state with the lowest possible energy. In quantum electrodynamics, this vacuum is referred to as QED vacuum to distinguish it from the vacuum of quantum chromodynamics.