master key vs passe-partout

master key and passe-partout both are nouns.

Word	Noun	Adjective	Verb	Adverb
master key	Yes	No	No	No
passe-partout	Yes	No	No	No

As nouns, master key and passe-partout are synonyms defined as:

Other synonyms of master key include master, passkey.

master key (noun)	passe-partout (noun)
key that secures entrance everywhere	key that secures entrance everywhere
	a mounting for a picture using gummed tape