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]

Moby Thesaurus II by Grady Ward, 1.0:

120 Moby Thesaurus words for "lighthouse":
   Klaxon, Mayday, SOS, Texas tower, air-raid alarm, alarm,
   alarm bell, alarm clock, alarm signal, alarum, alert, all clear,
   antenna tower, barbican, beacon, belfry, bell tower, belvedere,
   bench mark, bleachers, blinking light, bookmark, bridge,
   burglar alarm, buzzer, cairn, campanile, catstone, colossus,
   column, conning tower, crostarie, cupola, derrick, direction, dome,
   fiery cross, fire alarm, fire bell, fire flag, fire tower,
   five-minute gun, flashing light, fog bell, fog signal, foghorn,
   gale warning, gallery, gazebo, grandstand, guidance, hooter, horn,
   hue and cry, hurricane warning, landmark, lantern, lightship,
   lookout, loophole, mark, marker, martello, martello tower, mast,
   menhir, milepost, milestone, minaret, monument, note of alarm,
   obelisk, observation post, observation tower, observatory,
   occulting light, outlook, overlook, pagoda, peanut gallery,
   peephole, pharos, pilaster, pillar, pinnacle, platform, pole,
   police whistle, pylon, pyramid, ringside, ringside seat, seamark,
   shaft, sighthole, signal of distress, siren, skyscraper,
   small-craft warning, spire, standpipe, steeple, still alarm,
   storm cone, storm flag, storm warning, stupa, television mast,
   tocsin, top gallery, tope, tour, tower, turret, two-minute gun,
   upside-down flag, watchtower, water tower, whistle,
   windmill tower



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.