A distortion wave is a high-energy phenomenon which can be produced inside subspace rifts.
In 2370, while attempting to rescue the USS Fleming, which had become stranded in such a rift, the USS Enterprise encountered a series of distortion waves inside the rift, severely threatening structural integrity. Becoming stranded inside the rift themselves, Lieutenant Commander Geordi La Forge proposed the idea of phase-matching the Enterprise's deflector shield to the EM variance of the distortion waves to use the momentum of the waves to "surf" out of the rift. Although their initial attempt failed, they were eventually successful when they increased velocity, thereby lowering the speed differential between the wave and the Enterprise. (TNG: "Force of Nature")