In Star Trek: The Next Generation, warp nacelles create the warp field around a starship.
Warp nacelles are the long external engine structures mounted on starships such as the Enterprise-D. Their warp coils generate the field that surrounds and propels a vessel through subspace, allowing faster-than-light travel without ordinary movement through space at light speed.
The nacelles work with the warp core, which supplies the energy produced by the matter–antimatter reaction. The core provides power, while the nacelles shape that power into the warp field. Both components are essential, but they perform different jobs.
Warp nacelles are often confused with impulse engines, which provide sublight propulsion, or the deflector dish, which clears debris from a ship's path. Their paired placement also helps maintain the geometry required for a stable warp field on many Federation designs.