letter of intent vs missive

letter of intent and missive both are nouns.

Word	Noun	Adjective	Verb	Adverb
letter of intent	Yes	No	No	No
missive	Yes	No	No	No

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

letter of intent: any letter expressing an intention to take (or forgo) some action
missive: a written message addressed to a person or organization

Other hypernyms of letter of intent include letter.

letter of intent (noun)	missive (noun)
any letter expressing an intention to take (or forgo) some action	a written message addressed to a person or organization