petroleum

Etymology

From Medieval Latin petroleum, from petra (“rock”) + oleum (“oil”).

noun

1. A flammable liquid ranging in color from clear to very dark brown and black, consisting mainly of hydrocarbons, occurring naturally in deposits under the Earth's surface.
   petroleum-based products
   Coal and petroleum do not occur hap-hazard in nature, but the aggregate occurrence of each product in any region forms a definite, limited horizon, which, in its continuance, is independent of the bedding plane. 1940, William Plotts, Isogeotherm, Or Monist Theory of Stratified Mineral Occurrence and Origin, page 7