therm (thûrm)n.1. a. A unit of heat equal to 100,000 British thermal units (1.054 × 108 joules).b. A unit of heat equal to 1,000 large calories.2. a. A unit of heat equal to the large calorie.b. A unit of heat equal to the small calorie.[Greek therm?, heat, from thermos, warm, hot; see gwher- in Indo-European roots.]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 (???m) n (Units) Brit a unit of heat equal to 100 000 British thermal units. One therm is equal to 1.055 056 × 108 joules[C19: from Greek therm? heat]Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014therm (??rm) n. any of several units of heat, as one equivalent to 1000 large calories or 100,000 British thermal units. [1885?90;