/ Dictionary / Index S selection rule: Meaning and Definition of Find definitions for: selec'tion rule" Pronunciation: [key] — Physics. Physics. any of several rules designating allowed transitions between quantum states in terms of the quantum numbers associated with the states. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. selection selective Related Content Daily Word Quiz: paradox Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language