/ Dictionary / Index T thermionic tube: Meaning and Definition of Find definitions for: therm'ion'ic tube' Pronunciation: [key] — Electronics. Electronics. a vacuum tube in which the cathode is heated electrically to cause the emission of electrons by thermal agitation. Also called Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. thermionics thermionic valve 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