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.