/ Dictionary / Index I instability line: Meaning and Definition of Find definitions for: instabil'ity line" Pronunciation: [key] — Meteorol. Meteorol. a nonfrontal line of convective activity in the atmosphere, usually several hundred miles long but of relatively brief duration. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. instability instable Related Content Daily Word Quiz: sequester Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language