What is Mayday Call in Flight ?

A Mayday radio call should be reserved for life threatening situations. These may include, but are not limited to:

Loss, or imminent loss of aircraft control for any number of different reasons
aircraft upset by turbulence;
pilot incapacitation;
spatial disorientation;
control surface or structural failure;
engine failure that will lead to a forced landing/ditching/ejection/bailout;

Or, an onboard fire.