/ Dictionary / Index B boundary line: Meaning and Definition of Find definitions for: bound'ary line" Pronunciation: [key] boundary (def. 1). See Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. boundary layer boundary rider Related Content Daily Word Quiz: sinister Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language