Holy See
Although the Holy See is sometimes metonymically referred to as the "Vatican", the Vatican City State was distinctively established with the Lateran Treaty (1929) between the Holy See and Italy to ensure the temporal, diplomatic, and spiritual independence of the Papacy. As such, ambassadors are officially accredited to the Holy See and not the Vatican City State. Conversely, Papal nuncios to states and international organisations are recognised as representing the Holy See and the integrity of the Catholic Church along with its 1.3 billion members, not the Vatican City State, as prescribed also in the Canon law of the Catholic Church (1983). The "Holy See" thus refers to the See of Rome viewed as the central government of the Catholic Church. The Catholic Church, in turn, is the largest non-government provider of education and health care in the world, while the diplomatic status of the Holy See facilitates the access of its vast international network of charities.
Vatican City
The name "Vatican" was already in use in the time of the Roman Republic for a marshy area on the west bank of the Tiber across from the city of Rome. Under the Roman Empire, many villas were constructed there, after Agrippina the Elder (14 BC – 18 October AD 33) drained the area and laid out her gardens in the early 1st century AD. In AD 40, her son, Emperor Caligula (31 August AD 12–24 January AD 41; r. 37–41) built in her gardens a circus for charioteers (AD 40) that was later completed by Nero, the "Circus Gaii et Neronis", usually called, simply, the Circus of Nero.