In hydrologic terms, a measure of pressure at a given point in a liquid in terms of the vertical height of a column of the same liquid which would produce the same pressure

Refers to the method used to measure waterproofness. For example, if a fabric can stop a 5 meter column of water penetrating then it has a hydrostatic head of 5 meters, or 5000mm.

The height of a column of liquid. The difference in height between two points in a body of liquid.

the pressure at a given point in a liquid measured in terms of the vertical height of a column of the liquid needed to produce the same pressure

The pressure exerted by a column of fluid; i.e., the unit weight of the fluid times the vertical height of the column of that fluid.

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.