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A plural of spectrum.
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
the plural of spectrum
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014
n., pl. -tra (-trə), -trums.
a. an array of entities, as light waves or particles, ordered in accordance with the magnitudes of a common physical property, as wavelength or mass.
b. the band or series of colors, together with invisible extensions, produced by dispersion of radiant energy, as by a prism.
2. a broad range of varied but related ideas, objects, etc., that form a continuous series or sequence: the spectrum of political beliefs.
[1605–15; < Latin: appearance, form <spec(ere) to look, regard]
Random House Kernerman Webster's College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.