In fluid dynamics, head refers to the constant right hand side in the incompressible steady version of Bernoulli's equation. It is possible to express head in either units of height (e.g. meters) or in units of pressure such as pascals (the SI unit).
This is best understood by considering a waterwheel: the head is the vertical distance from the top of the waterwheel to the free surface of the millpond.
More generally, when considering a flow, one says that head is lost if energy is dissipated, usually through turbulence. In the context of steam trains, one talks of a good head of steam, referring to the pressure in the boiler.
The static head of a pump is the maximum height (pressure) it can deliver. The capacity of the pump can be read from its Q-H curve (flow vs height).
