A well drilled through impermeable strata into strata that receive water from a higher altitude so there is pressure to force the water to flow upward.
Well that is supplied with water rising naturally from an underground water-saturated rock layer (aquifer). The water rises from the aquifer under its own pressure. Such a well may be drilled into an aquifer that is confined by impermeable rocks both above and below. If the water table (the top of the region of water saturation) in that aquifer is above the level of the well head, hydrostatic pressure will force the water to the surface.
Artesian wells are often overexploited because their water is fresh and easily available, and they eventually become unreliable. There is also some concern that pollutants such as pesticides or nitrates can seep into the aquifers.
The name comes from Artois (Latin Artesium), a historical region of N France, on the Strait of Dover, where the phenomenon was first observed.