A steady flow is one in which all conditions at any point in a stream remain constant with respect to time.
A steady flow is the one in which the quantity of liquid flowing per second through any section, is constant.
This is the definition for the ideal case. True steady flow is present only in Laminar flow. In turbulent flow, there are continual fluctuations in velocity. Pressure also fluctuate at every point. But if this rate of change of pressure and velocity are equal on both sides of a constant average value, the flow is steady flow. The exact term use for this is mean steady flow.
Steady flow may be uniform or non-uniform.
A truly uniform flow is one in which the velocity is same at a given instant at every point in the fluid.
This definition holds for the ideal case. Whereas in real fluids velocity varies across the section.
But when the size and shape of cross section are constant along the length of channels under consideration, the flow is said to be uniform.
A non-uniform flow is one in which velocity is not constant at a given instant.
A flow in which quantity of liquid flowing per second is not constant, is called unsteady flow.
Unsteady flow is a transient phenomenon. It may be in time become steady or zero flow. For example when a valve is closed at the discharge end of the pipeline. Thus, causing the velocity in the pipeline to decrease to zero. In the meantime, there will be fluctuations in both velocity and pressure within the pipe.
Unsteady flow may also include periodic motion such as that of waves of beaches. The difference between these cases and mean steady flow is that there is so much deviation from the mean. And the time scale is also much longer.