The pressure, measured in pounds per square inch absolute (psia), exerted by a liquid, as determined by ASTM D323, Standard Method of Test for Vapor Pressure of Petroleum Products (Reid Method).
API RP 500, Recommended Practice for Classification of Locations for Electrical Installations at Petroleum Facilities Classified as Class I, Division 1 and Division 2, Third Edition, December 2012, Global Standards
For a given temperature, the pressure that maintains a liquid and its vapor in equilibrium.
Source: IADC UBO / MPD Glossary, December 2011. Global Standards