Vacuum

Vacuum, abbreviated as VAC, is a contained space having little or no matter or a volume having a pressure lower than the outside atmospheric pressure.  A perfect vacuum is a theoretical state in which there are no particles at all, not even photons, which are particles of light.  However, this state cannot be achieved in practice because there will always be some residual gas molecules, electromagnetic radiation, or other forms of energy present.

Vacuum has many important applications in physics, including in the study of quantum mechanics, particle physics, and cosmology.  For example, particle accelerators use high vacuum systems to accelerate particles to high energies, while astronomical telescopes use vacuum chambers to simulate the conditions of outer space.  Vacuum technology also has practical applications in various fields, such as in electronic devices, materials science, and vacuum insulation.