well Shaft sunk vertically in the Earth's crust through which water, oil, natural gas, brine, sulphur or other mineral substances can be extracted. Artesian wells are sunk into water-bearing rock strata, the aquifers, from which water rises under pressure in the wells to the surface.

well A completed borehole. The hole may be ‘dry’ in that it does not produce oil or gas (although containing water), or a producing well.