WordNet (r) 3.0 (2006):

Mayday
    n 1: an internationally recognized distress signal via
         radiotelephone (from the French m'aider)

Moby Thesaurus II by Grady Ward, 1.0:

44 Moby Thesaurus words for "Mayday":
   Klaxon, 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, signal of distress, siren, small-craft warning,
   still alarm, storm cone, storm flag, storm warning, tocsin,
   two-minute gun, upside-down flag, whistle