A geological formation that contains water, especially water that supplies springs and wells.

Aquifers have become important in petroleum marketing operations because gasoline and other products from leaking underground tanks and piping can find their way into an aquifer that serves as the drinking water source for a community.

A sole-source aquifer is an aquifer that has been designated by the U.S. Environmental Protection Agency as one that constitutes the only, or principal, water supply source for a city or other community.