1. A sequence of colours produced by passing light through a prism or diffraction grating. 2. A range of wavelengths of electromagnetic radiation. 3. A plot that shows how some intensity-related property of a beam of radiation or particles depends on another property that is related to dispersal of the beam by a prism, a magnet, or some other device. For example, a plot of light absorbance vs. wavelength is an absorption spectrum; a plot of ion abundance vs. mass is a mass spectrum.