/ Dictionary / Index S Supplemental Security Income: Meaning and Definition of Find definitions for: Supplemen'tal Secu'rity In"come Pronunciation: [key] income provided by the U.S. government to needy aged, blind, and disabled persons. Abbr.: SSI Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. supplemental plumage supplementary Related Content Daily Word Quiz: paradox Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language