The Collaborative International Dictionary of English v.0.48:

klaxon \klaxon\ n.
   A kind of loud horn formerly used on motor vehicles.

   Syn: claxon.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

klaxon
    n 1: a kind of loud horn formerly used on motor vehicles [syn:
         klaxon, claxon]

Moby Thesaurus II by Grady Ward, 1.0:

63 Moby Thesaurus words for "Klaxon":
   Mayday, SOS, air-raid alarm, alarm, alarm bell, alarm clock,
   alarm signal, alarum, alert, all clear, beacon, blinking light,
   boiler factory, boiler room, bull-roarer, burglar alarm, buzzer,
   catcall, cherry bomb, clack, clacker, clapper, cracker, cricket,
   crostarie, fiery cross, fire alarm, fire bell, fire flag,
   firecracker, five-minute gun, flashing light, fog bell, fog signal,
   foghorn, gale warning, hooter, horn, hue and cry,
   hurricane warning, lighthouse, noisemaker, note of alarm,
   occulting light, police whistle, rattle, rattlebox,
   signal of distress, siren, small-craft warning, snapper,
   steam whistle, still alarm, storm cone, storm flag, storm warning,
   ticktack, tocsin, two-minute gun, upside-down flag, whistle,
   whizgig, whizzer