/ Dictionary / Index D direct sum: Meaning and Definition of Find definitions for: direct' sum' Pronunciation: [key] — Math. Math. a composition of two disjoint sets, as vector spaces, such that every element in the composition can be written uniquely as the sum of two elements, one from each of the given sets. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. directrix direct tax Related Content Daily Word Quiz: diurnal Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language