encyclical letter vs missive

encyclical letter and missive both are nouns.

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

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

encyclical letter: a letter from the pope sent to all Roman Catholic bishops throughout the world
missive: a written message addressed to a person or organization

Other hypernyms of encyclical letter include letter.

encyclical letter (noun)	missive (noun)
a letter from the pope sent to all Roman Catholic bishops throughout the world	a written message addressed to a person or organization