Hydraulic fracturing: A process that stimulates or increases production and ultimate recovery from a well by pumping a fluid and a proppant (sand or similar material) at high pressure into a well to create fractures in the reservoir that the proppant holds open. Hydraulic fracturing increases the surface of the formation available for oil and natural gas to flow into the wellbore (the hole drilled into the earth to extract oil and natural gas). Water is the most commonly used fluid, but carbon dioxide and nitrogen are sometimes used. The proppant is usually sand, but resin-coated sand, ceramics, and sintered bauxite are sometimes used. Hydraulic fracturing is common in low-permeability formations.