A rotunda (from Latinrotundus) is any building with a circular plan, and sometimes covered by a dome. It may also refer to a round room within a building. A famous example is the one below the dome of the United States Capitol in Washington, D.C..
Further readings
Vera, Gervers-Molnár (1972): A középkori Magyarország rotundái. (Rotunda in the MedievalHungary). Akadémiai, Budapest
József, Csemegi (1949): A tarnaszentmáriai templom hajójának stíluskritikai vizsgálata. (Studies on the Nave of the Church at Tarnaszentmária.) in: Antiquitas Hungarica III (1949), 92-107.