A system in which at least one important performance parameter varies nonlinearly -- i.e., the slope of the performance curve for such a parameter is not constant. In a nonlinear system, the performance at any operating point cannot be linearly scaled to any other operating point. As such, the analysis of nonlinear systems is much more difficult than for linear systems. Analysis can sometimes be simplified by assuming that the system response is linear over a limited range.
Examples of nonlinear systems include buckling, electrical oscillations of circuits containing iron core inductors, and mechanical systems with nonlinear restoring forces. Most properties of piezoelectric materials are nonlinear.