The New Student's Reference Work/Spectrum

Spec′trum.  If a prism or a diffraction-grating be interposed in the path of a beam of white light, it will, provided the beam of white light be small, spread this beam out into a colored band in which the colors are arranged in the following order: red, orange, yellow, green, blue, violet.  This colored band is called the spectrum of white light.

If, instead of a pencil of white light, we had passed through the prism a pencil of light from the Bunsen flame, it too would have been separated into its constituent colors, in this case green, blue and violet, which are the spectrum of the Bunsen flame.  If an electric arc had been employed, we should have obtained the spectrum of the electric arc, even though a large part of the radiation from the arc is invisible—has no color—and must be detected by the photographic plate.

So we define the spectrum of any body as the ensemble of its radiations visible and invisible, arranged in the order of their wave-lengths.  See Spectroscopy.