From the Latin petra oleum, meaning "stone oil", an inflammable oily liquid varying in color from yellow to black, consisting of widely varying hydrocarbons, found in sedimentary strata of the earth's crust
EESiFlo North America, Terminology and Glossary: Oil & Gas Terms, Pennsylvania, USA 2002. flowmeterdirectory.com/flowmeter_terminology, (Mrtm/ Oil and Gas - Amer) {566}