Non-Linearity

Many systems display non-linear behaviour i.e. their outputs are not directly proportional to their inputs.
Non-linear systems do not satisfy superposition principal, or superposition property.
For all linear systems, net response at given place and time caused by two or more stimuli is sum of responses which would have been caused by stimuli individually.
If input A produces response X and input B produces response Y, input (A+B) produces response (X+Y).
System can also be non-linear if there is significant complexity that needs to be understood in order to understand the system.