Vacuum is
space void of
matter. The word stems 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
partial vacuum to refer to an actual imperfect vacuum as one might have in a
laboratory or in
space. In engineering and applied physics on the other hand, vacuum refers to any space in which the pressure is lower than atmospheric pressure. The Latin term
in vacuo is used to describe an object as being in what would otherwise be a vacuum.