1) Curve 2) Curved shape 3) The exterior curve of an arch 1) Exteriority 
• (n.) The exterior curve of an arch; esp., the upper curved face of the whole body of voussoirs. See Intrados.
The convex curve formed by the outer edge of an arch. The inner edge is called the intrados. 
In an arch or vault is the surface the top surface of the arch barrel. The outer (convex) curve of an arch
outer curve of the voussoirs. See arch. (Illustration) 
Outer curve of an arch with a structural or purely decorative function (* arch).
A construction line denoting outer limit of the arch. It is used in the setting out.; 
Ex·tra'dos noun [ F.; prefix extra- outside + dos (L. dorsum ) the back.] (Architecture) The exterior curve of an arch; esp., the upper curved face of the whole body of voussoirs. See Intrados . 
In architecture, an extrados is the exterior curve of an arch; especially, the upper curved face of the whole body of voussoirs.
the upper or outer curve of an arch. Often contrasted with intrados
The upper and / or outer vertical surfaces of arches and vaults. Also see: intrados.
[n] - the exterior curve of an arch
noun the exterior curve of an arch
the exterior curve or surface of an arch or vault. Also calledCf. intrados. See diag. under