Natural zones below the surface that yield water in sufficient quantities to be economically important for industrial, agricultural, or domestic purposes. Aquifers can occur in a variety of geologic materials, ranging from glacial-deposited outwash to sedimentary beds of limestone and sandstone, and fractured zones in dense igneous rocks. Composition and characteristics are almost infinite in their variety.

Aquifers can be confined or unconfined. Unconfined aquifers are those where direct contact can be made with the atmosphere, while confined aquifers are separated from the atmosphere by impermeable materials. Confined aquifers are also artesian aquifers. Though originally artesian was a term applied to water in an aquifer under sufficient pressure to produce flowing wells, the term is now generally applied to all confined situations.