the.com/flying buttress

means An external arched support that props up a wall or roof from outside, carrying the sideways thrust of a vaulted ceiling safely down to the ground.

from From French arc-boutant — 'bouter' meant to thrust or push, so the structure is literally named for what it does: shove back against a wall's outward lean. 'Flying' was tacked on in English because the arch leaps across open air to reach its pier, touching nothing below — a visibly airborne prop. Medieval cathedral builders used them to let walls grow taller and thinner, their windows wider, while the real muscle work happened outside, unseen by anyone praying inside.