Proven reserves, also called proved reserves, measured reserves, 1P, and Reserves, are business or political terms regarding fossil fuel energy sources. (see ). They are defined as a "Quantity of energy sources estimated with reasonable certainty, from the analysis of geologic and engineering data, to be recoverable from well established or known reservoirs with the existing equipment and under the existing operating conditions." These terms relate to common fossil fuel reserves such as oil reserves (see main article), natural gas reserves, or coal reserves.