An aquifer trapped between impermeable layers, holding water under pressure.
A confined aquifer (also called an artesian aquifer) sits between layers of low-permeability rock or clay that hold the water under pressure. Wells drilled into a confined aquifer often have a static water level much higher than the top of the producing zone, and in some cases water flows to the surface without pumping. Confined aquifers tend to be more protected from surface contamination than unconfined ones.