/ Dictionary / Index E eff.: Meaning and Definition of Find definitions for: eff. Pronunciation: [key] efficiency. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. See also: eff (Thesaurus) ef- effable Related Content Daily Word Quiz: endear Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language