voussoir

from WordNet (r) 3.0 (2006)

voussoir
    n 1: wedge-shaped stone building block used in constructing an
         arch or vault

from The Collaborative International Dictionary of English v.0.48

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.
   [1913 Webster]
   [1913 Webster]