2.45  spectrum. (a) Description, for a function of time, of the resolution of a signal into components, each of different frequency and (usually) different amplitude and phase. (b) “Spectrum” is also used to signify a continuous range of components, usually wide in extent, within which waves have some specified common characteristic; e.g., “audio frequency spectrum.”

Annotation 1          The term “spectrum” is also applied to functions of variables other than time, such as distance.

Annotation 2                  As defined here, “spectrum” defines a Fourier analysis of a signal or noise of a waveform. Basis functions other than sinusoids can be used, providing spectra other than Fourier.

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