A nonlinear device is a device which does not have a linear input/output relation. In a diode, for example, the current is a non-linear function of the voltage:
Most semiconductor devices have non-linear characteristics, but if the input of a non-linear device is expected to vary only in a small range around a fixed value, then the input/output relation can be linearized around this fixed value (usually called the quiescent point or Q-point or Bias Point).
