flying buttress vs buttressing

flying buttress and buttressing both are nouns.

Word	Noun	Adjective	Verb	Adverb
flying buttress	Yes	No	No	No
buttressing	Yes	No	No	No

As nouns, buttressing is a hypernym of flying buttress; that is, buttressing is a word with a broader meaning than flying buttress:

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

Other hypernyms of flying buttress include buttress.

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