discussion vs dialogue

discussion and dialogue both are nouns.

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

As nouns, dialogue is a hyponym of discussion; that is, dialogue is a word with a more specific, narrower meaning than discussion:

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