session vs group discussion

session and group discussion both are nouns.

Word	Noun	Adjective	Verb	Adverb
session	Yes	No	No	No
group discussion	Yes	No	No	No

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

session: a meeting for execution of a group's functions
group discussion: a discussion among participants who have an agreed (serious) topic

Other hypernyms of session include conference.

session (noun)	group discussion (noun)
a meeting devoted to a particular activity	a discussion among participants who have an agreed (serious) topic
a meeting for execution of a group's functions
a meeting of spiritualists
the time during which a school holds classes