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
"Drillers extracted vast amounts of petroleum from beneath the crust to fuel the nation's growing industrial needs."
In plain English: Petroleum is crude oil found underground that people refine into gasoline and other useful products.
"The price of gasoline rose because crude petroleum supply is low this winter."
Usage: Petroleum refers specifically to the natural crude oil found underground before it is refined into products like gasoline or heating fuel. Do not use this term for any processed fuels; instead, specify "gasoline," "kerosene," or other derivatives once they have been extracted and treated.
The word petroleum comes from Medieval Latin, combining petra meaning "rock" and oleum meaning "oil." It is a doublet of the word petrol.