/ Dictionary / Index I instantaneous: Meaning and Definition of Find definitions for: in•stan•ta•ne•ous Pronunciation: (in"stun-tā'nē-us), [key] — adj. occurring, done, or completed in an instant: an instantaneous response. existing at or pertaining to a particular instant: the instantaneous position of the rocket. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. See also: instantaneous (Thesaurus) instant instantaneous sound pressure Related Content Daily Word Quiz: acquiesce Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language