intrados

from WordNet (r) 3.0 (2006)

intrados
    n 1: the interior curve of an arch

from The Collaborative International Dictionary of English v.0.48

Intrados \In*tra"dos\, n. [F., fr. L. intra within + F. dos the
   back, L. dorsum. Cf. {Extrados}.] (Arch.)
   The interior curve of an arch; esp., the inner or lower
   curved face of the whole body of voussoirs taken together.
   See {Extrados}.
   [1913 Webster]