Mayday vs distress signal

Mayday and distress signal both are nouns.

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

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

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

Other hypernyms of Mayday include distress call.

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