From Longman Dictionary of Contemporary Englishflying buttressˌflying ˈbuttress noun [countable] AAa curved line of stones or bricks that are joined to the outside wall of a large building such as a church, and help to support it
Examples from the Corpus
flying buttress• A flying buttress transmits thrust rather then resisting it.• All are lofty with high vaults supported by flying buttress schemes. 