/ Dictionary / Index A ad init.: Meaning and Definition of Find definitions for: ad init. Pronunciation: [key] ad initium. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. ad infinitum ad initium Related Content Daily Word Quiz: pedagogy Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language