From Italian for "round", it's a large & high circular hall in a building, usually surmounted by a dome
Aisle: 
Subsidiary space alongside the body of a building, separated from it ... Basilica: 
Originally a Roman, large roofed hall erected for transacting business and 
disposing... [DOC] [TLE] Rotunda - definition of rotunda by The Free Dictionary [PAR] [Italian rotonda, from feminine of rotondo, round, from Latin rotundus; see rotund.]