In Fluid dynamics, the following four coefficients

are known as Hydraulic Coefficients. They are

**Coefficient of Contraction**

This is the ratio of the area of the jet at Vena Contracta to the area of the orifice.

**Coefficient of Velocity**

This is the ratio of the actual velocity of the jet at vena contracta to the theoretical velocity of the jet

**Coefficient of resistance**

This is the ratio of loss of head in the orifice to the actual head. This loss of head is due to the frictional losses when the fluid passes through the orifice.

**Coefficient of Discharge**

This is the ratio of the actual discharge to the theoretical discharge.