mass meeting vs assembly

mass meeting and assembly both are nouns.

Word	Noun	Adjective	Verb	Adverb
mass meeting	Yes	No	No	No
assembly	Yes	No	No	No

As nouns, assembly is a hypernym of mass meeting; that is, assembly is a word with a broader meaning than mass meeting:

Other hypernyms of mass meeting include assemblage, gathering.

mass meeting (noun)	assembly (noun)
a large gathering of people intended to arouse enthusiasm	the social act of assembling
	the act of constructing something (as a piece of machinery)
	a group of machine parts that fit together to form a self-contained unit
	a public facility to meet for open discussion
	a unit consisting of components that have been fitted together
	a group of persons who are gathered together for a common purpose