Spectrum
The result of dispersing a beam of electromagnetic radiation so that components with different wavelengths are separated in space, in order of increasing or decreasing wavelength. The most familiar example of spectrum is the natural rainbow, which occurs when sunlight is dispersed into its component colours by the combined prism-like action of raindrops. The full spectrum of electromagnetic radiation encompasses, in order of decreasing wavelength, radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-radiation, and gamma-radiation. (Mitton, 1993, Dictionary of Astronomy, London: Penguin Books)