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, carré du and transept, croisée du (synonyms).
For all parts of the transept described above see also all other entries related to transept.
|Carré du transept - Wikipedia|