A vacuum is a space devoid of matter.The word is derived from the Latin adjective vacuus for "vacant" or "void".An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often discuss ideal test results that would occur in a perfect vacuum, which they sometimes simply call "vacuum" or free space, and use the term …

The effects of vacuum energy can be experimentally observed in various phenomena such as spontaneous emission, the Casimir effect and the Lamb shift, and are thought to influence the behavior of the Universe on cosmological scales.Using the upper limit of the cosmological constant, the vacuum energy of free space has been estimated to be 10 −9 joules (10 −2 ergs), …