/ Dictionary / Index A add-in: Meaning and Definition of Find definitions for: add-in Pronunciation: (ad'in"), [key] a component, as an expansion card or chip, added to a computer to expand its capabilities. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. See also: add-in (Thesaurus) Addie adding machine Related Content Daily Word Quiz: blanch Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language