A Mayday call is an internationally recognized distress signal used primarily by aviators and mariners to signal a life-threatening emergency. It is used to indicate that an aircraft or vessel is in grave and imminent danger and requires immediate assistance. The term "Mayday" was conceived as a distress call in the early 1920s by Frederick Stanley Mockford, officer-in-charge of radio at Croydon Airport, England. He was asked to think of a word that would indicate distress and would easily be understood by all pilots and ground staff in an emergency. Since much of the air traffic at the time was between Croydon and Le Bourget Airport in Paris, he proposed the term "mayday," the phonetic equivalent of the French maidez ("help me") or maider (a short form of venez maider, "come .

The Mayday distress signal is said three times in a row, "Mayday! Mayday! Mayday!" to avoid confusion with other words or phrases that sound similar under noisy conditions. If a Mayday call cannot be sent because a radio is not available, a variety of other distress signals and calls for help can be used. In the United States, civilian aircraft making a Mayday call are encouraged by the Federal Aviation Administration to use a specific format that includes the aircraft call sign, nature of emergency, weather, pilots intentions or requests, present position and heading, altitude, fuel remaining, and number of people on board.

It is important to note that a Mayday call is not something to be taken lightly, and it is illegal to make a fake distress call in the United States. Doing so can result in jail time and a hefty fine.