expansion vs discussion

expansion and discussion both are nouns.

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

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

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

Other hypernyms of expansion include discourse, treatment.

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