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.