What is???MAYDAY!MAYDAY!THIS IS JULLIET ALFA....111...?

WE HAVE AN EMERGENCY!!!
what is actually mayday?what is its origin?where is used?

i was discussing this matter with my coleagues
we forgot what is the 3rd expression beside mayday and pan-pan.we need your help!thanks a lot for the effort!

is an emergency code word used internationally as a distress signal in voice procedure radio communications, derived from the French venez m'aider, meaning "come [to] 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.

It is dervied from the French: venez m'adier , which means come help me.
It is an international distress call.
It is used in aviation. I think it is used for marine emergencies also.
It is used for emergencies only. It is repeated three times, to eliminate any confusion: May-dayMay-dayMay-day.
There is another one that is used for urgent situations that may lead to emergencies. Pan-pan,Pan-pan,Pan-pan.