/ Dictionary / Index J J particle: Meaning and Definition of Find definitions for: J particle Pronunciation: [key] — Physics. Physics. an early name for the J/psi particle. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. JP Jpn. Related Content Daily Word Quiz: fabricate Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language