distress call vs Mayday

distress call and Mayday both are nouns.

Word	Noun	Adjective	Verb	Adverb
distress call	Yes	No	No	No
Mayday	Yes	No	No	No

As nouns, Mayday is a hyponym of distress call; that is, Mayday is a word with a more specific, narrower meaning than distress call:

distress call: an internationally recognized signal sent out by a ship or plane indicating that help is needed
Mayday: an internationally recognized distress signal via radiotelephone

Other hyponyms of distress call include SOS.

distress call (noun)	Mayday (noun)
an internationally recognized signal sent out by a ship or plane indicating that help is needed	an internationally recognized distress signal via radiotelephone