Coaxial warp drive is an advanced propulsion method that folds space to move a vessel instantly between distant points, encountered by Voyager aboard a ship flown by the alien Steth.
Unlike conventional warp, coaxial warp folds space so a ship can jump to a destination almost immediately. Voyager's crew helped an alien pilot named Steth when his experimental coaxial drive malfunctioned, and Tom Paris became fascinated by the design, spending hours helping Steth repair it.
Steth turned out to be a genome-stealing alien who swapped bodies with Paris and took his place aboard Voyager. The deception was eventually exposed and the swap reversed.