Why Is Transfer Function Defined for Linear Systems Only?

[srinaath]

i have few queries regarding control system in electrical engineering...
1)transfer function is defined for linear system ? why is that so...
2)why transfer function is defined with zero initial condition...why not with initial condition

 

[LvW]

A transfer function is the output-to-input signal ratio. Such a ratio can be defined for equal signals forms only.
(Can you divide a triangle form by a sinus signal?).
The output signal form is equal to the form of the input signal for linear systems only.