Source : Webster's Revised Unabridged Dictionary (1913)

Searchlight \Search"light`\, n.
   (a) An apparatus for projecting a powerful beam of light of
       approximately parallel rays, usually devised so that it
       can be swiveled about.
   (b) The beam of light projecting by this apparatus.

Source : WordNet®

searchlight
     n : a light source with reflectors that projects a beam of light
         in a particular direction