flying buttress vs arc-boutant

flying buttress and arc-boutant both are nouns.

Word	Noun	Adjective	Verb	Adverb
flying buttress	Yes	No	No	No
arc-boutant	Yes	No	No	No

As nouns, flying buttress and arc-boutant are synonyms defined as:

flying buttress and arc-boutant: a buttress that stands apart from the main structure and connected to it by an arch

flying buttress (noun)	arc-boutant (noun)
a buttress that stands apart from the main structure and connected to it by an arch	a buttress that stands apart from the main structure and connected to it by an arch