/ Dictionary / Index T tachyon: Meaning and Definition of Find definitions for: tach•y•on Pronunciation: (tak'ē-on"), [key] — n. Physics. a hypothetical particle that travels faster than the speed of light. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. tachymeter tachyphylaxis Related Content Daily Word Quiz: diligent Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language