The Collaborative International Dictionary of English v.0.48:

Lighthouse \Light"house`\ (l[imac]t"hous`), n.; pl.
   Lighthouses (l[imac]t"houz`[e^]z).
   A tower or other building with a powerful light at top,
   erected at the entrance of a port, or at some important point
   on a coast, to serve as a guide to mariners at night; a
   pharos.
   [1913 Webster]

WordNet (r) 3.0 (2006):

lighthouse
    n 1: a tower with a light that gives warning of shoals to
         passing ships [syn: beacon, lighthouse, beacon light,
         pharos]

The Devil's Dictionary (1881-1906):

LIGHTHOUSE, n.  A tall building on the seashore in which the
government maintains a lamp and the friend of a politician.