Having had a number of situations flying jets and twin props I would add that there are often grey areas which are not white or black.
Obviously if the Donkey goes bang it is a straight mayday situation.
I had an emergency sitauation in a citation flying from Biggin Hill to Majorca at night. climbing through FL240 over the channel The first indication was a slight burning smell. I was asked to contact the french controllers but requested I remain with London for a little longer as there may be a problem.
Smoke then started coming out of the air vents. Priority then was to go to the emergency checklist to try and contain the problem and work out a course of action.
There followed communication with ATC who asked about the nature of the problem and my desired intentions.
At no time did ATC refer to a possible Mayday but instead used the terminology of "declaring an emergency". In other occasions when there has been a problem some which have been rectified or worked around there has always been ongoing communication with ATC and never once have I heard ATC refer to "are you declaring a Mayday" but ALWAYS "are you declaring an emergency". So not even ATC seem that committed to the word Mayday.
In the above ATC Placed a precautionary emergency in place and gave me a direct to Biggin with a continuous descent.
Mayday goes back a long time and always brings to mind wartime films with some poor guys wing falling off his spitfire.
Usually the situation is not clear cut and develops to a situation where an emergency is declared and usually with some length of communication with ATC.
Pace