Meaning of flying buttress

fly'ing but'tress

Pronunciation: [key]
— Archit. Archit.
  1. a segmental arch transmitting an outward and downward thrust to a solid buttress that through its inertia transforms the thrust into a vertical one. See illus. under
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.