noun pl. spectra -·tra (-trə) or spectrums -·trums

1. the series of colored bands dispersed and arranged in the order of their respective wavelengths by the passage of white light through a prism or other dispersing device and shading continuously from red (produced by the longest wave visible) through violet (produced by the shortest): the six main colors of the spectrum are red, orange, yellow, green, blue, and violet, with a seventh color (indigo) sometimes specified, between blue and violet
2. the intensity of any radiation or motion displayed as a function of frequency, or wavelength
3. an afterimage
4. a continuous range or entire extent a wide spectrum of opinion
5. 1. radio spectrum
   2. electromagnetic spectrum

Etymology: ModL, special use (by Sir Isaac Newton, 1671) of L spectrum: see specter