Rotonde (rotunda)

A circular space in the heart of a church, meant to facilitate processions of big groups of pilgrims.
The picture shows the interior of the Chiesa di Santo Stefano Rotondo in Rome; the plan of Viollet le Duc (Wikipedia) underneath shows the rotunda in the crypte (crypt) of the Saint-Bénigne in Dijon.

Chiesa di Santo Stefano Rotondo - Rome

Rotonde - Wikipedia