voussoir

noun

vous·​soir vü-ˈswär 

ˈvü-ˌswär

: one of the wedge-shaped pieces forming an arch or vault see arch illustration

Word History

Etymology

French, from Old French vosoir, from Vulgar Latin *volsorium, from Latin volvere to roll — more at wallow entry 1

First Known Use

1728, in the meaning defined above