I am a native French speaker, and a translator.
I'd like to make a short terminology comparison.
In English, for architecture, use arch, as in "pointed arch", in all cases. For geometry, use arc, or bow, like in bow-window.
In French, arc's first meaning is "bow" (cf. archer).
Arche (Fr) is used to describe a large, primarily bow structure, typically in a bridge or a large space. But Arc (Fr) is just generic or for smaller construction elements. Pointed arch e.g. is said Arc Brisé (and not Arche). And for geometry as well it is Arc.
On Paris' Champs Élysees, you have the Arc de Triomphe, Rome has the Arc de Titus, which both are rather Arches (Fr) really by their dimensions, but for that kind of monument Arc is the traditionally used terminology.
London has the Marble Arch.
And finally rainbow is said Arc-en-ciel (litt. a bow in the sky). How nice is that !
I hope this can somehow help & that no one will be upset this little linguistic invasion of mine.