/ Dictionary / Index T third sector: Meaning and Definition of Find definitions for: third' sec'tor Pronunciation: [key] the segment of a nation's economy that is made up of neither public nor business concerns, as nonprofit health or educational institutions. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. Third Republic third stream Related Content Daily Word Quiz: ounterfeit Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language