ROTUNDA, or ROTUNDO, a term applied to a building which is circular in its plan, both externally and internally. The technical application of the word is restricted to circular buildings whose height does not greatly exceed their diameter. The most celebrated rotunda of the ancients is the Pantheon at Rome, of which the external diameter is 188 feet; internal, 142; internal height, 142. The Radcliffe Library at Oxford is a polygon of sixteen sides and 104 feet in diameter; exterior height, 140 feet; interior diameter, 88 feet; interior height, 90 feet.