Bernoulli's theorem states that when an incompressible and non-viscous liquid (or gas) flows in stream-lined motion from one place to another then at every point in its path the total energy per unit volume
(i.e., pressure energy + kinetic energy + potential energy) is constant.
That is P+21ρv2+ρgh= constant
Thus, Bernoulli's theorem is in one form, the principle of conservation of energy for-a flowing liquid (or gas).
Dividing the Bemoulli's equation be ρg. we have ρgP+2gv2+h= constant
In this expression ρgP is called the 'pressure head', 2gv2 the 'velocity head' and h the 'gravitational head'.