open letter vs missive

open letter and missive both are nouns.

Word	Noun	Adjective	Verb	Adverb
open letter	Yes	No	No	No
missive	Yes	No	No	No

As nouns, missive is a hypernym of open letter; that is, missive is a word with a broader meaning than open letter:

open letter: a letter of protest; addressed to one person but intended for the general public
missive: a written message addressed to a person or organization

Other hypernyms of open letter include letter.

open letter (noun)	missive (noun)
a letter of protest; addressed to one person but intended for the general public	a written message addressed to a person or organization