Wikipedia Reference Information
Mayday is an emergency code word used internationally as a distress signal in voice procedure radio communications, derived from the French m'aider, meaning "help me." It is used to signal a life-threatening emergency by many groups, such as police forces, pilots, the fire brigade, and transportation organizations. 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.
The complete, up-to-date and editable article about Mayday can be found at Wikipedia: Mayday