It is an emergency procedure word used internationally as a distress signal in voice procedure radio communications. It derives from the French venez m'aider, meaning "come help me".
It is used to signal a life-threatening emergency primarily by mariners and aviators but in some countries local organisations such as police forces, firefighters, and transportation organizations may also use the term. The call is always given three times in a row ("Mayday Mayday Mayday") to prevent mistaking it for some similar-sounding phrase under noisy conditions, and to distinguish an actual Mayday call from a message about a Mayday call.
A mayday situation is one in which a vessel, aircraft, vehicle, or person is in grave and imminent danger and requires immediate assistance. Examples of "grave and imminent danger" in which a mayday call would be appropriate include fire, explosion or sinking.
Civilian aircraft in the UK and Europe are encouraged to use the following format:
MAYDAY, MAYDAY, MAYDAY; Name of station addressed; Aircraft callsign; Nature of emergency; Intentions of the pilot; Present position (including Altitude or Flight level); Pilot's qualifications; Any other useful information (number of souls on board.)
MAYDAY, MAYDAY, MAYDAY, this is NONSUCH, NONSUCH, NONSUCH. MAYDAY, NONSUCH. Position 54 25 North 016 33 West. My boat is on fire and sinking. I require immediate assistance. Four people on board, are taking a lifeboat. MAYDAY NONSUCH, OVER."