Source : Webster's Revised Unabridged Dictionary (1913)

Voussoir \Vous`soir"\, n. [F., akin to vo[^u]te an arch, a
   vault.] (Arch.)
   One of the wedgelike stones of which an arch is composed.

Source : WordNet®

voussoir
     n : wedge-shaped stone building block used in constructing an
         arch or vault