/ Dictionary / Index B B.T.U.: Meaning and Definition of Find definitions for: B.T.U. Pronunciation: [key] — Physics. Physics. British thermal unit; British thermal units. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. See also: btu (Thesaurus) btry. bu. Related Content Daily Word Quiz: miscreant Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language