Mayday vs distress call

Mayday and distress call both are nouns.

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

As nouns, distress call is a hypernym of Mayday; that is, distress call is a word with a broader meaning than Mayday:

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

Other hypernyms of Mayday include distress signal.

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