The transwarp threshold (also known as the maximum warp barrier or the transwarp barrier) was a boundary that separated warp from transwarp and was identified in the 24th century as the velocity of warp 10.
In 2372, Tom Paris crossed the transwarp threshold while attempting to devise a means to help his ship, the USS Voyager, hasten its journey back to the Alpha Quadrant. (VOY: "Threshold")