dialogue vs discussion

dialogue and discussion both are nouns.

Word	Noun	Adjective	Verb	Adverb
dialogue	Yes	No	No	No
discussion	Yes	No	No	No

As nouns, discussion is a hypernym of dialogue; that is, discussion is a word with a broader meaning than dialogue:

Other hypernyms of dialogue include give-and-take, word.

dialogue (noun)	discussion (noun)
the lines spoken by characters in drama or fiction	an exchange of views on some topic
a conversation between two persons	an extended communication (often interactive) dealing with some particular topic
a literary composition in the form of a conversation between two people
a discussion intended to produce an agreement