1. Also called sonic barrier, transonic barrier. (not in technical use) a hypothetical barrier to flight beyond the speed of sound, so postulated because aircraft undergo an abruptly increasing drag force induced by compression of the surrounding air when traveling near the speed of sound.
2. break the sound barrier, to travel faster than the speed of sound.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.