visa vs imprimatur

visa and imprimatur both are nouns.

Word	Noun	Adjective	Verb	Adverb
visa	Yes	No	Yes	No
imprimatur	Yes	No	No	No

As nouns, imprimatur is a hypernym of visa; that is, imprimatur is a word with a broader meaning than visa:

visa: an endorsement made in a passport that allows the bearer to enter the country issuing it
imprimatur: formal and explicit approval

Other hypernyms of visa include countenance, endorsement, indorsement, sanction, warrant.

visa (noun)	imprimatur (noun)
an endorsement made in a passport that allows the bearer to enter the country issuing it	formal and explicit approval

visa (verb)	imprimatur (verb)
provide (a passport) with a visa
approve officially