Nacelle
From Memory Alpha, the free Star Trek reference.
A nacelle is an outboard structure on starships which houses the warp coils, necessary components of warp flight. The warp coils create a subspace displacement field, which "warps" the space around the vessel allowing it to "ride" on a spatial distortion, and travel faster than the speed of light. (ENT: "Cold Front") While not always present on starships, warp nacelles are the most common component of warp flight, dating as far back as Zefram Cochrane's original warp ship, the Phoenix, circa 2063.
Aboard Federation vessels of the 24th century, warp coils are fed by plasma conduits from the warp core reactor assembly. Nacelles are usually separated from the main structure of the ship because of radiation generated by the nacelles; when at optimal levels, the radiation could be deleterious to the safety of ship and crew. Nacelles are separated from the ship by large pylons, and usually house a Bussard ramscoop at the fore end for the purpose of collecting hydrogen fuel to replenish the impulse engines.
A nacelle's interior also includes a small control room, accessible in nominal conditions by a Jefferies tube that permits maintenance and monitoring of the system's operation. (TNG: "Eye of the Beholder") Aboard the prototypical NX class Enterprise of the 22nd century, a long catwalk spanned the length of each nacelle and, in emergency situations, acted as shelter for the ship's crew against certain types of radiation such as that created by neutronic storms. (ENT: "The Catwalk", "The Crossing")
Most vessels typically have two nacelles. However, vessels can operate with one nacelle disabled, but at reduced warp speeds. (VOY: "Year of Hell", ENT: "Twilight") It is not unprecedented, though, for vessels to have additional nacelles. One example would be Federation Constellation-class starships, which have four nacelles. (TNG: "The Battle")
In an unknown, alternate future timeframe, a refit version of the USS Enterprise-D was equipped with a third nacelle. (TNG: "All Good Things...")
Even at sublight speeds serious impacts from weapons or other objects can spell disaster for a ship because of a feedback of energy throughout the vessel. The Enterprise-D was destroyed by such an impact in an alternate timeline when the USS Bozeman collided with one of Enterprise's warp nacelles. (TNG: "Cause and Effect")
Some starship classes, such as the Defiant-class and the Steamrunner-class, possess nacelles that are integrated into the ship's main structure without pylons. (Star Trek: Deep Space Nine; Star Trek: First Contact).
[edit] Background
The use of the word nacelle in spacecraft design descends from its use for similar housing structures in air and water craft design.
The Star Trek: The Next Generation Technical Manual states that having two nacelles is the optimal configuration for vessel control and efficiency. It also gives information on a separation system in the event that the ship is damaged and unable to retain the nacelle safely on the pylon - explosive charges separate the nacelle and lift it up and away at 30 meters per second - and goes on to note that if a nacelle is lost during warp flight, it can tear the ship apart, as the loss of a nacelle would cause different areas of the ship to travel at different speeds.
