Frequency response is defined as the ratio of the phasor output to the phasor input where the output and input may be either a voltage or a current. The most common ratio is phasor output voltage to phasor input voltage.
H(
) is often referred to as the voltage transfer function. You may
refer to H(
) as either the frequency response or the
voltage transfer function.
Figure 
shows a comparison between the frequency response
of a 1st order (contains 1 energy storage element) physical lowpass filter and an
ideal lowpass filter.
Figure: Comparison of the frequency responses of an ideal lowpass filter and a 1st order
physical approximation.
