/ Dictionary / Index S supplemental plumage: Meaning and Definition of Find definitions for: sup'plemen'tal plum'age Pronunciation: [key] — Ornith. Ornith. the third plumage assumed by certain birds having three different plumages in their annual cycle of molts. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. supplemental Supplemental Security Income Related Content Daily Word Quiz: feral Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language