fancy-dress ball vs masked ball

fancy-dress ball and masked ball both are nouns.

Word	Noun	Adjective	Verb	Adverb
fancy-dress ball	Yes	No	No	No
masked ball	Yes	No	No	No

As nouns, fancy-dress ball and masked ball are synonyms defined as:

fancy-dress ball and masked ball: a ball at which guests wear costumes and masks

Other synonyms of fancy-dress ball include masquerade ball.

fancy-dress ball (noun)	masked ball (noun)
a ball at which guests wear costumes and masks	a ball at which guests wear costumes and masks