Rotunda (architecture)

A rotunda (from Latin rotundus) is any building with a circular ground plan, and sometimes covered by a dome. It may also refer to a round room within a building (a famous example being the one below the dome of the United States Capitol in Washington, D.C.). The Pantheon in Rome is a famous rotunda. A band rotunda is a circular bandstand, usually with a dome.
en.wikipedia.org