The range of frequencies present in a waveform, such as an audio signal. When a time-varying signal is subjected to frequency analysis, it is transformed from the time domain to the frequency domain. The frequency-domain representation of the signal is called the spectrum, and the time domain representation is called the waveform. The two quantities contain the same information, and one can be converted into the other by a mathematical operation of the Fourier transform. Spectrum analysis, frequency analysis, and Fourier analysis are synonymous.