The Collaborative International Dictionary of English v.0.48:

Tocsin \Toc"sin\, n. [F., fr. OF. toquier to touch, F. toquer
   (originally, a dialectic form of F. toucher) + seint (for
   sein) a bell, LL. signum, fr. L. signum a sign, signal. See
   Touch, and Sign.]
   An alarm bell, or the ringing of a bell for the purpose of
   alarm.
   [1913 Webster]

         The loud tocsin tolled their last alarm. --Campbell.
   [1913 Webster]

WordNet (r) 3.0 (2006):

tocsin
    n 1: the sound of an alarm (usually a bell) [syn: tocsin,
         alarm bell]
    2: a bell used to sound an alarm [syn: tocsin, warning bell]

Moby Thesaurus II by Grady Ward, 1.0:

46 Moby Thesaurus words for "tocsin":
   Klaxon, Mayday, SOS, air-raid alarm, alarm, alarm bell,
   alarm clock, alarm signal, alarum, alert, all clear, beacon,
   blinking light, burglar alarm, buzzer, crostarie, fiery cross,
   fire alarm, fire bell, fire flag, five-minute gun, flashing light,
   fog bell, fog signal, foghorn, gale warning, hooter, horn,
   hue and cry, hurricane warning, lighthouse, note of alarm,
   occulting light, police whistle, sign, signal, signal of distress,
   siren, small-craft warning, still alarm, storm cone, storm flag,
   storm warning, two-minute gun, upside-down flag, whistle