The Pantheon ( or US ; Latin: Pantheon, from Greek: Πάνθειον, an adjective meaning "to every god") is a building in Rome, commissioned by Marcus Agrippa as a temple to all the gods of Ancient Rome, and rebuilt by Emperor Hadrian in about 126 CE. The nearly-contemporary writer (2nd–3rd centuries CE), Cassius Dio, speculated that the name comes either from the statues of so many gods placed around this building, or else from the resemblance of the dome to the heavens. Since the French Revolution, when the church of Sainte-Geneviève, Paris, was deconsecrated and turned into a secular monument, the Panthéon of Paris, the generic term pantheon has been sparsely applied to any building in which the illustrious dead are honored or buried.
The building is circular with a portico of three ranks of huge granite Corinthian columns (eight in the first rank and two groups of four behind) under a pediment opening into the rotunda, under a coffered, concrete dome, with a central opening (oculus) to the sky. Almost two thousand years after it was built, the Pantheon's dome is still the world's largest unreinforced concrete dome. The height to the oculus and the diameter of the interior circle are the same, 43.3 metres (142 ft). A rectangular structure links the portico with the rotunda. It is one of the best preserved of all Roman buildings. It has been in continuous use throughout its history, and since the 7th century, the Pantheon has been used as a Roman Catholic church dedicated to "St. Mary and the Martyrs" but informally known as "Santa Maria Rotonda"; the square in front of the Pantheon is called Piazza della Rotonda.