Abside (apse)

This is the part of the church that closes off the chœur (choir); in plan (Wikipedia) the abside is indicated in grey.
The plan of an apse is often semicircular, sometimes polygonal.
On the photo the abside of the Saint-Gervais-et-Saint-Protais in Bourg-le-Comte.
This apse is decorated with bandes lombardes (Lombard bands).

Abside - Wikipedia

Saint-Gervais et Saint-Protais - Bourg-le-Comte