The term 'Zipangu' refers to an old name of an island which was said to be Japan in Europe.
It is said that the word of Zipangu was derived from the pronunciation of 'Nippon koku or Nihon koku' (日本国, literally meaning country of Japan) in Middle-Age Chinese, and was first introduced to Europe by Marco Polo as Cipangu or Chipangu. A rhyming dictionary, "Zhongyuan Yinyun (中原音韻,literally, Sounds and Rhymes of the Central Plains)," complied in the late Yuan Dynasty (in 1324) enabled the pronunciation of the characters, 日本国, in Proto-Mandarin Chinese before and after the Yuan Dynasty to be collated with the pronunciation of '日 (ni)' as riət (entering tone), '本 (pon or hone)' as puən (rising tone), and '国 (koku)' as kuo (entering tone). Meanwhile, "the Menggu-Ziyun (蒙古字韻, Mongolian Letters arranged by Rhyme)" edited by Zhu Zongwen (朱宗文) in late Great Yuan Dynasty (in 1308), which was a rhyming dictionary of Chinese ideographs arranged according to phonetic syllables written in the Phags-pa script, categorized '日' as the entering tone of ži, and '本' as the rising tone of bun, and '国' as entering tone of guų respectively.
In the modern pinyin system, the word becomes rì-běn-guó, pronouncing 'ri-ben-guo.'
Incidentally, around from the tenth century, Waqwaq (الواقواق al-Wāqwāq, 倭国 [Wakoku [Japan]]?) was often referenced in some geography books written in Arabic and Persian languages, including the one written by geographer, Ibn Khurdādh-Bih, as an island (a country) with gold mines, which later evolved into the legend of Zipangu, the land of gold.
Many words for Japan in modern languages, such as Japan, Japon, Giappone, and Yaponiya, are generally derived from the word 'Zipangu,' but there are also different theories, including one theory holding that the origin of these words was that people in Southeast Asia called Japan Japang in a borrowed word from Chinese language around the 16th century, when Portuguese reached Southeast Asia.
In Japan, the fact that Marco Polo introduced Japan as Zipangu to Europe is well known, so that Zipangu is thought to be another name of Japan.
Zipangu which Marco Polo wrote
"The million: Travels of Marco Polo" by Marco Polo wrote as follows:
Zipangu was an independent island country, which was located 1,500 miles across the sea to the east of Catai (an alternative name for the Chinese continent). It produced a large quantity of gold, so that palaces and private houses were made in gold, being abundant in treasure. People on the island worshiped idols and were good-looking and polite, but they had a man-eating custom.
Kublai of Mongolia sent a military expedition to Japan to conquer it, but that expedition failed after a huge storm scattered the Mongol fleet completely. Mongolian soldiers left behind alive on the island occupied the capital of Zipangu during the unguarded moment when Zipangu's soldiers were absent from it and resisted against them, but they made peace under a condition that they were granted to live in this country.
Zipangu' in the Mongolian Empire
What can be said about information on Japan documented during the Mongolian Empire period, or the Yuan Dynasty, is that the subsection on Japan, 'Japan Accounts' (日本伝), of the 'History of Song Dynasty,' which was complied in the late Great Yuan Dynasty by Prime Minister Toktoghan recounted twenty-three generations from Amenominakanushi-no-mikoto to Amaterasu Omikami (the Sun Goddess) as 'what the Japanese chronicle had listed' and about sixty-four sovereigns, such as from the Emperor Jinmu (a grandson of Amaterasu Omikami) to the Emperor Reizei and the subsequent Emperor Enyu (the Emperor Morihira), who was the emperor of that age (the early Song Dynasty), and recorded as follows:
The land extended several thousand 'ri,' (ri [里] is about 3.927km)' from northeast to southwest respectively, and was bounded by the sea on the southwest, and the border of northeast was a huge mountain. '
Beyond the mountain, there was a country of Emishi people (the Ezo [northerners], and the Ainu).'
The Five Classics texts of Confucianism, Buddhist scriptures, and seventy volumes of Juyi BAI's poems, all of which had been obtained from China, were possessed all over the country.'
The land was fertile in five grains, but it produced a little wheat.'
Silkworms were farmed for raw silk, by which a large quantity of silk, fine and fabulous, was weaved.'
The subsection also referred to 'the climate of four seasons (spring, summer, fall, and winter).'
Additionally, this 'Japan Accounts' included the almost correct information on Japan's geography; for example, gold was mined in the 'Oshu region' in the east, and silver in another island in the west of Japan, which appeared to be Tsushima Island although it referred to partly different descriptions from true fact, including the one that there were many rhinoceroses and elephants in Japan.
Also, the 'History of the Mongols,' a subsection of the "Compendium Chronicle (Jami al-Tawarikh)," stated that there was a large island named Jimangu (جمنكو Jimangū?), which was surrounded by a great ocean, located to the southeast, 400 farsang (or about 2,000km; farsang is an Iranian or Persian unit of measurement for how far a man can walk in one day) off the coast of the Jurchen and Goryeo (جورجه و كولى Jūrja wa Kūlī) regions.
It included the description that a large island located far away across the sea from Jurchen and Goryeo as an area against the Great Yuan Dynasty, transcribed as 'جمنكوj-m-n-k-w,' which appeared to be a 'country of Japan.'
As mentioned above, at the present day, it is generally believed that Zipangu told by Marco Polo referred to Japan, but there are some different opinions.
In Medieval times, Japan rather imported gold, which was in contradiction to the legend of Zipangu, a land of gold.
The description of Zipangu given by Marco Polo and other legends apparently located it in the Torrid Zone (both its degrees of latitude and its climate), which was quite different from the actual location of Japan (it belongs the Temperate Zone).
There existed many other countries than Japan where Mongolian expedition resulted in failure.
For these reasons, one theory holds that it was a misunderstanding of missionaries who reached Japan during the 16th century who connected Zipangu with Japan. Additionally, another one explains that the origin of Zipangu was from a dialect of ツィァパングォ (foreign lands) which was indicated as small countries in Southeast Asia, where the Yuan Dynasty sent a military expedition. Moreover, another is that although WaqWaq is said to have been derived from the old name for Japan, 'wakoku (倭国),' with the Japanese pronunciation 'waakuwaaku' (الواقواق al-Wāqwāq), or 'wakuwaaku' (لوقواق al-Waqwāq), which spread in Muslim's world (the Arabian and Persian speaking areas) as the land of gold, an area name of its kind was often found in Africa and Southeast Asia in geography books written in Arabia and Persian languages, so that it didn't indicate Japan.