Source : Webster's Revised Unabridged Dictionary (1913)

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}.

Source : WordNet®

intrados
     n : the interior curve of an arch