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]