Arête (groin vault)

A crossing of two voûtes en berceau (barrel vaults) of equal height.
An arête (in general a sharp edge) is the cutting line of two barrel vaults crossing each other perpendicular.
The sketch from Wikipedia hardly needs any more explanation.
In the picture a groin vault in the Saint-Philibert in Tournus.
See also voûte d'arête (synonym) and voûte en berceau.

Arête - Wikipedia

Saint-Philibert - Tournus