A vaccuum is a volume of space that is essentially empty of matter, so that gaseous pressure is much less than standard atmospheric pressure. The root of the word vaccuum is the Latin adjective vacuus which means "empty," but space can never be perfectly empty. A perfect vaccuum with a gaseous pressure of absolute zero is a philosophical concept that is never observed in practice, not least because quantum theory predicts that no volume of space is perfectly empty in this way. Physicists often use the term "vaccuum" slightly differently. They discuss ideal test results that would occur in a perfect vaccuum, which they simply call "vaccuum" or "free space" in this context, and use the term partial vaccuum to refer to the imperfect vacua realized in practice.