A linear element refers to an element whose output and input have a proportional relationship. For example, when the temperature is constant, the relationship between the voltage at both ends of the metal resistance element and the current can be considered to be linear. Electronic components have many of these relationships. Poor quality components exhibit "linear distortion". Non-linear components do not have a "linear" relationship between their input and output. When the signal passes through an electronic component, the waveform of the signal does not change, we call it a linear device; such as a resistor, a capacitor.