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.