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Related to population inversion: laser
The condition of having enough excited or high-energy states distributed throughout a substance to sustain a chain-reaction of stimulated emission. Lasers, for example, require population inversion in order to generate radiation.
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.