Definition. The phase response
of an LTI
filter is defined as the phase (or complex angle) of the frequency
response
:

The phase response is often referred to as the ``phase'' of the
filter. For filters with real coefficients, the filter phase
can be defined unambiguously as the phase of the frequency response of
the filter.

The real-valued phase response
gives the phase
shift in radians that each input component sinusoid will undergo.