1. In astronomy, an energetic knot of gas in a galactic jet (large-scale aggregates of stars, gas, and dust that constitute the universe) which has a high velocity and is going in a direction such that it gives the appearance of having faster-than-light movements.
2. Pertaining to speeds faster than that of shining machines: As a child, Priscilla was always puzzled that she could see jet planes in the sky long before she could hear the roaring sounds of their motors; later, she learned that the loud noises were not superluminal sounds because they were never able to be heard before they could be seen.