voussoir
Concise Oxford English Dictionary © 2008 Oxford University Press:
voussoir /ˈvuːswɑː/
▶noun Architecture a wedge-shaped or tapered stone used to construct an arch.
– origin C18: via Fr. from pop. L. volsorium, based on L. volvere ‘to roll’.
'voussoir' also found in these Oxford entries:

