The pressure exerted by a body of water at rest. The hydrostatic head of fresh water is 0.433 psi per foot of height. Those of other liquids may be determined by comparing their gravities with the gravity of water. See pressure gradient.
When generating hydropower, the head is a general term used to describe the distance that a given water source has to fall before the point where power is generated. Ultimately the force responsible for hydropower is gravity, so a hydroelectricity plant with a tall/high head can produce more energy than a similar plant with a short/low head. The difference is immediately apparent if you compare the Hoover Dam in Colorado with the Aswan Dam in Egypt.