therm-pref. Variant of thermo-.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.therm (??rm) n. any of several units of heat, as one equivalent to 1000 large calories or 100,000 British thermal units. [1885?90;