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An empty space without any matter in it. The term is derived from the Latin “vacuus”, meaning “vacant” or “void”. Because a perfect vacuum without any particles is difficult to achieve, a vacuum is usually referred to as a region in which the pressure is many times lower than atmospheric pressure. The lower the pressure, the better the vacuum. At pressures lower than 1 × 10−6 Pa we speak of an ultra-high vacuum. On Earth, it is created artificially by vacuum pumps. The extreme-high vacuum is in intergalactic space, which contains only a few particles per cubic meter. Electromagnetic waves can propagate in a vacuum.