Vacuum 8 : Theoretically, a space absolutely devoid of all matter and exerting zero pressure. However, vacuum is commonly used to describe a condition that exists in system when pressure is reduced below atmospheric pressure. (WLOP)
The Mudblogger (Oilfield, Energy and Related Photographs), Oil Field Glossary, TX, USA 2014. http://mudblogger.com/oilfield-glossary/, (Oil - Amer) {777}