affair vs social gathering

affair and social gathering both are nouns.

Word	Noun	Adjective	Verb	Adverb
affair	Yes	No	No	No
social gathering	Yes	No	No	No

As nouns, affair and social gathering are synonyms defined as:

affair and social gathering: a formal or official social gathering or ceremony of people

Other synonyms of affair include function, occasion, social affair, social function, social occasion.

affair (noun)	social gathering (noun)
a vaguely specified concern	a formal or official social gathering or ceremony of people
a formal or official social gathering or ceremony of people
a usually secretive or illicit sexual relationship