rotunda.
1. Building shaped like a cylinder both inside and outside, especially one covered with a dome, such as the Pantheon in Rome. It may have a peristyle around the exterior (e.g. the drum of the Panthéon crossingdome in Paris by Soufflot) or within it, or both inside and outside (e.g. tholos at Epidaurus).

2. Hall or room shaped like a cylinder contained within a larger building so that the drum is not expressed externally as a totality, but may appear partially as a bow, with or without a cupola over it.

ro·tun·da / rōˈtəndə/ • n. a round building or room, esp. one with a dome.