A system that is considered to be operating in a steady state although it may actually be an unsteady state process in which a process variable varies with time. An example is a vessel containing a viscous liquid that discharges through a small orifice. A steady-state material balance is conveniently used to describe the rate of discharge and change in capacity in the vessel although the progressive fall in level is dependent on the characteristics of the orifice. The validity of a quasi-steady-state mathematic model can be assessed by setting the time derivative to zero.