Source : Webster's Revised Unabridged Dictionary (1913)

Extrados \Ex*tra"dos\, n. [F.; pref. extra- outside + dos (L.
   dorsum) the back.] (Arch.)
   The exterior curve of an arch; esp., the upper curved face of
   the whole body of voussoirs. See {Intrados}.

Source : WordNet®

extrados
     n : the exterior curve of an arch