voussoir
One of a series of wedge-shaped bricks or stones forming an arch or vault.
It is the voussoir depth in a real arch which enables the arch to carry wider ranges of loading; a large number of different idealized centre-line arches can be contained within a given practical profile. ...[T]his must be so, or no mediaeval bridge would have survived its decentering.
Where it comes from
Unadapted borrowing from French voussoir, from Old French vosoir, from Vulgar Latin *volsorium, from *volsus, from Latin volvō (“to roll”).
Adapted from the Wiktionary entry voussoir (CC BY-SA 4.0).