An artesian aquifer is an aquifer whose water is overpressurized. Water will thus flow out of an artesian well without pumping.

Why is a well artesian?

Because its recharge zone is higher than the head of the well.

Pascal's Law predicts the overpressure:

 Poverpressure = ρg(zrecharge - zwellhead)

where ρ is the volumic mass of the fluid, g the acceleration due to gravity, and z is height.

Etymology

First recorded in 1830, from French puits artésien "wells of Artois," French province where such wells were first bored 18th century by Bélidor .