From WordNet (r) 3.0 (2006) [wn]:
lighthouse
n 1: a tower with a light that gives warning of shoals to
passing ships [syn: beacon, lighthouse, beacon light,
pharos]
From The Collaborative International Dictionary of English v.0.48 [gcide]:
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]