From a UK perspective only Mayday or Pan is recognised as an emergency call, however both pilots and ATC can decide if one should be declared. If an a/c loses 50% or more of its power then a Mayday will automatically be declared. ATC Towers have more catergories describing emergencies from local standbys to full emergencies to ground accidents, and they alone decide what to apply and dependent on which they choose decides if any or all the local flashing blue lights greet you at the runway threshold. This has been discussed at length many times before, a search of rumours and news and also ATC Issues will drag the threads up.