buttress vs arc-boutant

buttress and arc-boutant both are nouns.

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

As nouns, arc-boutant is a hyponym of buttress; that is, arc-boutant is a word with a more specific, narrower meaning than buttress:

buttress: a support usually of stone or brick; supports the wall of a building
arc-boutant: a buttress that stands apart from the main structure and connected to it by an arch

Other hyponyms of buttress include flying buttress.

buttress (noun)	arc-boutant (noun)
a support usually of stone or brick; supports the wall of a building	a buttress that stands apart from the main structure and connected to it by an arch

buttress (verb)	arc-boutant (verb)
make stronger or defensible
reinforce with a buttress