The Sufficient and necessary condition for a
surface to be called as Free Surface.
Its Answer is : Shear Stress acting on the surface must be Zero.
At free surface of water, there is a equal atmospheric pressure on the entire surface, surface tension which is tensile in nature will be present but no shear stress as at free surface, gradient of velocity is negligible, So, no shear stress acting on it must be Zero.
The term is very much important for the GATE Examination point of view and often asked.