A Warp nacelle is an outboard structure found on warp capable vessels. Vessels with warp drive will have two or more nacelle.
Warp nacelles contain a number of field coils which create a warp field when injected with highly energetic warp plasma, or plasma created in the warp core by the reaction of deuterium and antideuterium, mediated through an assembly of dilithium crystals, which are nonreactive with antimatter when subjected to high-frequency electromagnetic fields. The warp nacelle is connected to the warp core via plasma transfer conduits, which transferred the plasma in the core to the nacelles. Because the raditaion produced by a nacelle was detrimental to living beings, the nacelle was usually placed at some distance from the main structure of the vessel, and held in place with a pylon.
Studies by the United Federation of Planets found that two nacelles were the optimum number for creating the most efficient warp field, as well as for vessel control. (Star Trek: The Next Generation Technical Manual). However the Federation has fielded vessels with only one nacelle, such as the Saladin class, or three or more nacelles. (Star Fleet Technical Manual). In an anti-time future created by Q, the USS Enterprise-D was equipped with a third nacelle. (TNG episode: "All Good Things...")