discussion vs expansion

discussion and expansion both are nouns.

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

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

discussion: an extended communication (often interactive) dealing with some particular topic
expansion: a discussion that provides additional information

Other hyponyms of discussion include detail, dilation, consideration, talk, elaboration, enlargement.

discussion (noun)	expansion (noun)
an exchange of views on some topic	the act of increasing (something) in size or volume or quantity or scope
an extended communication (often interactive) dealing with some particular topic	adding information or detail
	a discussion that provides additional information
	a function expressed as a sum or product of terms