A round church is a church with a completely circular plan, thus a rotunda in architectural terms.
Østerlars Round Church, Bornholm, Denmark
Church of San Marcos in Salamanca
Church of the Holy Transfiguration, Sarajevo, Bosnia
Church of Saint Vitus, Rijeka, Croatia
A rotunda is any roofed building with a circular ground plan, and sometimes covered by a dome. It may also refer to a round room within a building. The Pantheon in Rome is perhaps the most famous, and is the most influential rotunda. A band rotunda is a circular bandstand, usually with a dome.
The Rotunda at the University of Virginia, designed by US President Thomas Jefferson, one of many based on the Pantheon, Rome
Temple of Hercules Victor, in the Forum Boarium in Rome
Remains of the circular temple at Bairat. A stupa was located in the center, with a colonnade and a circular wall around. 3rd century BCE.
Ashoka's circular Mahabodhi Temple, Bharhut