The Collaborative International Dictionary of English v.0.48:

Magellanic cloud \Mag`el*lan"ic cloud\, n. (Astron.)
   Either of two conspicuous celestial nebulae near the south
   celestial pole, resembling thin white clouds, each of which
   is a galaxy[2] smaller than but separate from the Milky Way
   galaxy, and together they are the galactic formations nearest
   to our galaxy. They are not visible from the northern
   hemisphere, and are named after Ferdinand Magellan, who saw
   them in his expedition, which passed through the Strait of
   Magellan in South America, and one ship of which completed
   the first circumnavigation of the globe.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Magellanic Cloud
    n 1: either of two small galaxies orbiting the Milky Way;
         visible near the south celestial pole