/ Dictionary / Index T thermal diffusion: Meaning and Definition of Find definitions for: ther'mal diffu'sion Pronunciation: [key] — Physical Chem. Physical Chem. the separation of constituents, often isotopes, of a fluid under the influence of a temperature gradient. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. thermal conductivity thermal efficiency 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