The Collaborative International Dictionary of English v.0.48:

foghorn \fog"horn\ n.
   A horn that emits a loud low-pitched sound, used on ships
   navigating in a fog, to warn other ships of their presence.

   Syn: fog signal.
        [PJC]

WordNet (r) 3.0 (2006):

foghorn
    n 1: a loud low warning signal that can be heard by fogbound
         ships [syn: foghorn, fogsignal]
    2: a warning device consisting of a horn that generates a loud
       low tone

Moby Thesaurus II by Grady Ward, 1.0:

111 Moby Thesaurus words for "foghorn":
   Klaxon, Mayday, Roman candle, SOS, aid to navigation,
   air-raid alarm, alarm, alarm bell, alarm clock, alarm signal,
   alarum, alert, all clear, amber light, balefire, beacon,
   beacon fire, bell, bell buoy, blinker, blinking light, blue peter,
   buoy, burglar alarm, buzzer, caution light, crostarie, fiery cross,
   fire alarm, fire bell, fire flag, five-minute gun, flare,
   flashing light, fog bell, fog signal, fog whistle, gale warning,
   glance, go light, gong buoy, green light, heliograph, high sign,
   hooter, horn, hue and cry, hurricane warning,
   international alphabet flag, international numeral pennant, kick,
   leer, lighthouse, marker beacon, nod, note of alarm, nudge,
   occulting light, parachute flare, pilot flag, poke, police whistle,
   quarantine flag, radio beacon, red flag, red light, rocket,
   sailing aid, semaphore, semaphore flag, semaphore telegraph, sign,
   signal, signal beacon, signal bell, signal fire, signal flag,
   signal gong, signal gun, signal lamp, signal light, signal mast,
   signal of distress, signal post, signal rocket, signal shot,
   signal siren, signal tower, siren, small-craft warning, spar buoy,
   still alarm, stop light, storm cone, storm flag, storm warning,
   the nod, the wink, tocsin, touch, traffic light, traffic signal,
   two-minute gun, upside-down flag, watch fire, whistle, white flag,
   wigwag, wigwag flag, wink, yellow flag