Olympiad
An Olympiad (, Olympiás) is a period of four years associated with the Olympic Games of the Ancient Greeks.  Although the Ancient Olympic Games were established during Archaic Greece, it was not until the Hellenistic period, beginning with Ephorus, that the Olympiad was used as a calendar epoch. Converting to the modern BC/AD dating system the first Olympiad began in the summer of 776 BC and lasted until the summer of 772 BC, when the second Olympiad would begin with the commencement of the next games.
By extrapolation to the Julian calendar, the
 year of the  th Olympiad began in (Northern-Hemisphere) mid-summer  .

A modern Olympiad refers to a four-year period beginning January 1 of the year the Olympic Summer Games are normally held. The first modern Olympiad began January 1, 1896, the second January 1, 1900, and so on (the 32nd began January 1, 2020: see the Olympic Charter).

The ancient and modern Olympiads would have synchronised had there been a year zero between the Olympiad of 4 BC and the one of 4 AD. But as the Julian calendar goes directly from 1 BC to 1 AD, the ancient Olympic cycle now lags the modern cycle by one year.

Ancient Olympics 
An ancient Olympiad was a period of four years grouped together, counting inclusively as the ancients did. Each ancient Olympic year overlapped onto two of our modern reckoning of BC or AD years, from midsummer to midsummer. Example: Olympiad 140, year 1 = 220/219 BC; year 2 = 219/218 BC; year 3 = 218/217 BC; year 4 = 217/216 BC. Therefore, the games would have been held in July/August of 220 BC and held the next time in July/August of 216 BC, after four olympic years had been completed.

Historians 
The sophist Hippias was the first writer to publish a list of victors of the Olympic Games, and by the time of Eratosthenes, it was generally agreed that the first Olympic games had happened during the summer of 776 BC.  The combination of victor lists and calculations from 776 BC onwards enabled Greek historians to use the Olympiads as a way of reckoning time that did not depend on the time reckonings of one of the city-states. (See Attic calendar.) The first to do so consistently was Timaeus of Tauromenium in the third century BC. Nevertheless, since for events of the early history of the games the reckoning was used in retrospect, some of the dates given by later historian for events before the 5th century BC are very unreliable.  In the 2nd century AD, Phlegon of Tralles summarised the events of each Olympiad in a book called Olympiads, and an extract from this has been preserved by the Byzantine writer Photius.  Christian chroniclers continued to use this Greek system of dating as a way of synchronising biblical events with Greek and Roman history.  In the 3rd century AD, Sextus Julius Africanus compiled a list of Olympic victors up to 217 BC, and this list has been preserved in the Chronicle of Eusebius.

Examples of Ancient Olympiad dates 

 Early historians sometimes used the names of Olympic victors as a method of dating events to a specific year.  For instance, Thucydides says in his account of the year 428 BC: "It was the Olympiad in which the Rhodian Dorieus gained his second victory".
 Dionysius of Halicarnassus dates the foundation of Rome to the first year of the seventh Olympiad, 752/1 BC. Since Rome was founded on April 21, which was in the last half of the ancient Olympic year, it would be 751 BC specifically. In Book 1 chapter 75 Dionysius states: "...Romulus, the first ruler of the city, began his reign in the first year of the seventh Olympiad, when Charops at Athens was in the first year of his ten-year term as archon."
 Diodorus Siculus dates the Persian invasion of Greece to 480 BC: "Calliades was archon in Athens, and the Romans made Spurius Cassius and Proculus Verginius Tricostus consuls, and the Eleians celebrated the Seventy-fifth Olympiad, that in which Astylus of Syracuse won the stadion. It was in this year that king Xerxes made his campaign against Greece."
 Jerome, in his Latin translation of the Chronicle of Eusebius, dates the birth of Jesus Christ to year 3 of Olympiad 194, the 42nd year of the reign of the emperor Augustus, which equates to the year 2  BC.

Start of the Olympiad 
An Olympiad started with the holding of the games, which occurred on the first or second full moon after the summer solstice, in what we call July or August. The games were therefore essentially a new years festival. In 776 BC this occurred on either July 23 or August 21. (After the introduction of the Metonic cycle about 432 BC, the start of the Olympic year was determined slightly differently).

Anolympiad 
Though the games were held without interruption, on more than one occasion they were held by others than the Eleians. The Eleians declared such games Anolympiads (non-Olympics), but it is assumed the winners were nevertheless recorded.

End of the era 
During the 3rd century AD, records of the games are so scanty that historians are not certain whether after 261 they were still held every four years. During the early years of the Olympiad, any physical benefit deriving from a sport was banned. Some winners were recorded though, until the last Olympiad of 393AD. In 394, Roman Emperor Theodosius I outlawed the games at Olympia as pagan. Though it would have been possible to continue the reckoning by just counting four-year periods, by the middle of the 5th century AD reckoning by Olympiads had become disused.

Modern Olympics

Start and end 
The Summer Olympics are more correctly referred to as the Games of the Olympiad. The first poster to announce the games using this term was the one for the 1932 Summer Olympics, in Los Angeles, using the phrase: Call to the games of the Xth Olympiad

The modern Olympiad is a period of four years. The first Olympiad started on 1 January 1896, consecutive Olympiads started (or will start) on 1 January of the years evenly divisible by four. This means that the count of the Olympiads continues even if Olympic Games are cancelled: For instance, the regular intervals would have meant (summer) Olympic Games should have occurred in 1940 and 1944; both were cancelled on account of WWII. Nonetheless, the count of the Olympiads continued: The 1936 Games were those of the XI Olympiad; the next summer games were those of 1948, which were the games of the XIV Olympiad. The current Olympiad is the XXXII of the modern era, which began on 1 January 2020.

Note, however, that the official numbering of the Winter Olympics does not count Olympiads—- it counts only the Games themselves. For example:
 The first Winter Games, in 1924, were not designated as Winter Games of the VII Olympiad, but as the I Winter Olympic Games.
 The 1936 Summer Games were the Games of the XI Olympiad. After the 1940 and 1944 Summer Games were canceled due to World War II, the Games resumed in 1948 as the Games of the XIV Olympiad.
 However, the 1936 Winter Games were the IV Winter Olympic Games, and the resumption of the Winter Games in 1948 was designated the V Winter Olympic Games.

Some media people have from time to time referred to a particular (e.g., the nth) Winter Olympics as "the Games of the nth Winter Olympiad", perhaps believing it to be the correct formal name for the Winter Games by analogy with that of the Summer Games. Indeed, at least one IOC-published article has applied this nomenclature as well. This analogy is sometimes extended further by media references to "Summer Olympiads". However, the IOC does not seem to make an official distinction between Olympiads for the summer and winter games, and such usage particularly for the Winter Olympics is not consistent with the numbering discussed above.

Quadrennium 
Some Olympic Committees often use the term quadrennium, which it claims refers to the same four-year period. However, it indicates these quadrennia in calendar years, starting with the first year after the Summer Olympics and ending with the year the next Olympics are held. This would suggest a more precise period of four years, but, for example, the 2001–2004 Quadrennium would then not be exactly the same period as the XXVIIth Olympiad.

Cultural Olympiad 

A Cultural Olympiad is a concept protected by the International Olympic Committee and may be used only within the limits defined by an Organizing Committee for the Olympic Games. From one Games to the next, the scale of the Cultural Olympiad varies considerably, sometimes involving activity over the entire Olympiad and other times emphasizing specific periods within it. Baron Pierre de Coubertin established the principle of Olympic Art Competitions at a special congress in Paris in 1906, and the first official programme was presented during the 1912 Games in Stockholm. These competitions were also named the ‘Pentathlon of the Muses’, as their purpose was to bring artists to present their work and compete for ‘art’ medals across five categories: architecture, music, literature, sculpture and painting.

Nowadays, while there are no competitions as such, cultural and artistic practice is displayed via the Cultural Olympiad. The 2010 Winter Olympics in Vancouver presented the Cultural Olympiad Digital Edition. The 2012 Olympics included an extensive Cultural Olympiad with the London 2012 Festival in the host city,  and events elsewhere including the World Shakespeare Festival produced by the RSC. The 2016 games' Cultural Olympiad was scaled back due to Brazil's recession; there was no published programme, with director Carla Camurati promising "secret" and "spontaneous" events such as flash mobs.
Cultural events in time for Tokyo 2020 are being planned.

Other uses 
The English term is still often used popularly to indicate the games themselves, a usage that is uncommon in ancient Greek (as an Olympiad is most often the time period between and including sets of games).  It is also used to indicate international competitions other than physical sports. This includes international science olympiads, such as the International Geography Olympiad, International Mathematical Olympiad and the International Linguistics Olympiad and their associated national qualifying tests (e.g., the United States of America Mathematical Olympiad or the United Kingdom Linguistics Olympiad), and also events in mind-sports, such as the Science Olympiad, Mindsport Olympiad, Chess Olympiad, International History Olympiad and Computer Olympiad. In these cases Olympiad is used to indicate a regular event of international competition for top achieving participants; it does not necessarily indicate a four-year period.

In some languages, like Czech and Slovak, Olympiad () is the correct term for the games.

The Olympiad (L'Olimpiade) is also the name of some 60 operas set in Ancient Greece.

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