/ Dictionary / Index E exclusion principle: Meaning and Definition of Find definitions for: exclu'sion prin"ciple Pronunciation: [key] — Physics. Physics. the principle that in any system described by quantum mechanics no two identical particles having spin equal to half an odd integer can be in the same quantum state: first postulated for the electrons in atoms. Also calledCf. fermion. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. exclusionism exclusive Related Content Daily Word Quiz: cygnet Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language