/ Dictionary / Index T thermal underwear: Meaning and Definition of Find definitions for: ther'mal un'derwear Pronunciation: [key] underwear designed to retain body heat in cold temperatures. Also called Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. thermal spring thermal unit 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