Extrados

from WordNet (r) 3.0 (2006)

extrados
    n 1: the exterior curve of an arch

from The Collaborative International Dictionary of English v.0.48

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}.
   [1913 Webster]