According to the presentation of the signal in the form of a generalizedFourier series, the spectrum is:

The spectrum of a signal represented as a generalized Fourier series is a representation of the signal's frequency content. It is often referred to as the frequency domain representation of the signal. The spectrum is obtained by taking the Fourier transform of the signal, which converts the time-domain signal into a frequency-domain signal. The resulting spectrum shows the frequency components of the signal and their respective amplitudes and phases. The spectrum can be used to analyze the frequency content of the signal, identify frequency components, and filter out unwanted frequencies.