social gathering vs affair

social gathering and affair both are nouns.

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

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

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

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

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