/ Dictionary / Index S set theory: Meaning and Definition of Find definitions for: set' the"ory Pronunciation: [key] the branch of mathematics that deals with relations between sets. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. setter setting Related Content Daily Word Quiz: spectral Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language