Critical flow is the flow at critical depth in uniform flow.

several conditions for critical flow are:

1)Specific Energy should be minimum

2)Discharge should be maximum

3)Froude number of that flow be unity

4)kinetic energy head is equal to D/2 where D=Hydraulic Depth

Critical depth can be found from the graph between depth vs Specific Energy for a constant discharge.

If all these cases are satisfied then it is called critical flow.