It is the overlapping area of the nef principale (central nave) and the transept.
The crossing is often covered with a coupole (cupola), which in itself forms the base of a clocher (bell tower).
The picture comes from Wikipedia; the crossing is indicated in grey.
See also transept, croisée du (synonym).
For all parts of the transept described above see also all other entries related to transept.
|Carré du transept - Wikipedia|