/ Dictionary / Index T thermal efficiency: Meaning and Definition of Find definitions for: ther'mal effi'ciency Pronunciation: [key] — Thermodynam. Thermodynam. the ratio of the work output of a heat engine to the heat input expressed in the same units of energy. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. thermal diffusion thermal equilibrium Related Content Daily Word Quiz: wizened Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language