skeleton key vs master key

skeleton key and master key both are nouns.

Word	Noun	Adjective	Verb	Adverb
skeleton key	Yes	No	No	No
master key	Yes	No	No	No

As nouns, master key is a hypernym of skeleton key; that is, master key is a word with a broader meaning than skeleton key:

skeleton key: a passkey with much of the bit filed away so that it can open different locks
master key: key that secures entrance everywhere

Other hypernyms of skeleton key include master, passe-partout, passkey.

skeleton key (noun)	master key (noun)
a passkey with much of the bit filed away so that it can open different locks	key that secures entrance everywhere