The Collaborative International Dictionary of English v.0.48:

Nacelle \Na*celle"\ (n[.a]*s[e^]l"), n. [F.]
   1. A small boat. [Obs.]
      [Webster 1913 Suppl.]

   2. The basket suspended from a balloon; hence, the framework
      forming the body of a dirigible balloon, and containing
      the machinery, passengers, etc.
      [Webster 1913 Suppl.]

   3. A streamlined enclosure on an airplane, as for the engine
      or for the cargo or passengers; -- formerly used to refer
      to the boatlike, inclosed body of an airplane which is
      usually now called the fuselage, and now referring
      mostly to the enclosure for the engine.
      [Webster 1913 Suppl. +PJC]

WordNet (r) 3.0 (2006):

nacelle
    n 1: a streamlined enclosure for an aircraft engine