voussoir
/vuːˈswɑː(r)/noun
- Architecture a wedge-shaped or tapered stone used to construct an arch【建筑】（楔形或锥形）拱石。
词源
early 18th cent: via French from popular Latin volsorium, based on Latin volvere 'to roll'. The word， borrowed from Old French， was also used for a time in late Middle English.