Webster's English Dictionary

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spectrum

spec.trum \'spek-tr*m\ \-tr*\ n or spec.tra or spectrums [NL, fr. L, 
   appearance - more at SPECTER] pl  1: an array of the components of an 
   emission or wave separated and arranged  in the order of some varying 
   characteristic (as wavelength, mass, or energy) : as 1a: a series of images 
   formed when a beam of radiant energy is subjected to  dispersion and 
   brought to focus so that the component waves are arranged in the order of 
   their wavelengths (as when a beam of sunlight that is refracted and 
   dispersed by a prism forms a display of colors) 1b: ELECTROMAGNETIC 
   SPECTRUM  1c: RADIO SPECTRUM  2: a continuous sequence or range