ultra high vacuum — Ultra high vacuum (UHV) is the vacuum regime characterised by pressures lower than about 10⁷ pascal or 100 nanopascals (~10⁹ torr). UHV requires the use of special materials in creating the vacuum system, extreme cleanliness to maintain the vacuum system, and baking the entire system to remove water and other trace gases that are accumulated due to the difficulties of maintaining an UHV.

References: Wikipedia, the free encyclopedia – http://en.wikipedia.org/wiki/Ultra_high_vacuum