The Bernoulli principle is built on the laws of conservation of energy and momentum. It says that the total work done on the fluid is constant, and therefore takes the form:

is called the

*dynamic pressure*, and represents the kinetic energy of the fluid due to the actual external forces acting on it i.e. the flow.

is the gravitational potential energy, taking into account any change in height of the flow.

P is the

*static pressure* and represents the pressure of the fluid excluding the kinetic energy due to the actual flow i.e. the dynamic pressure. For example, for a flow through a pipe, the static pressure would be the pressure felt over the wall of the pipe. Even though the fluid is flowing parallel to it, and therefore is not contributing any kinetic energy

*from the flow* (dynamic pressure), it still does contribute some kinetic energy as static pressure, just like a canister of pressurized gas.

It's accurate for incompressible streamlined flow, but applies intuitively to other flows too.

If the velocity increases, the dynamic pressure increases, and for a constant height, this must mean that the static pressure has to decrease since the energy must remain constant. The Venturi meter and Pitot-static tube both use this effect to measure the velocity of a fluid (An airplanes airspeed for example):

http://en.wikipedia.org/wiki/Pitot_tube
http://en.wikipedia.org/wiki/Venturi_meter