One or more valves installed at the wellhead to prevent the escape of pressure either in the annular space between the casing and the drill pipe or in an open hole (for example, hole with no drill pipe) during drilling or completion operations
O'Brien Energy Company, Oil and Gas Exploration and Production, Glossary, LA, USA 2014. http://www.obrienenergyco.com/glossary/, (Oil - Amer) {781}