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Myriad (Ancient Greek: μυριάς myrias, plural μυριάδες myriades), "numberless, countless, infinite", is a classical Greek word for the number 10,000. In modern English, the word refers to an unspecified large quantity.
History and usage 
A myriad is primarily a singular cardinal number; just as the "thousand" in "four thousand" is singular (one does not write "four thousands people") the word myriad is used in the same way: "there are four myriad people outside". When used as a noun, meaning "a large number", it follows the same rules as that phrase. However, that is not the case originally in Greek, where there is plural.
In English, the term "myriad" is most commonly used to refer to a large number of an unspecified size. In this way "myriad" can be used as either an adjective or a noun. Thus both "there are myriad people outside" and "there is a myriad of people outside" are correct.
Merriam-Webster notes, "Recent criticism of the use of myriad as a noun, both in the plural form myriads and in the phrase a myriad of, seems to reflect a mistaken belief that the word was originally and is still properly only an adjective.... however, the noun is in fact the older form, dating to the 16th century. The noun myriad has appeared in the works of such writers as Milton (plural myriads) and Thoreau (a myriad of), and it continues to occur frequently in reputable English."
Modern Greek still uses the word "myriad" by itself, but also to form the word for million. The word for million is ekatommyrio (hundred myriad — εκατομμύριο); one thousand million is disekatommyrio (twice hundred myriad — δισεκατομμύριο).
The largest number named in Ancient Greek was a myriad myriad and Archimedes of Syracuse used this quantity as the basis for a numeration system of large powers of ten, which he needed to count grains of sand, see The Sand Reckoner.
In Sweden and Norway, one mile = 10,000 metres = one myriametre. Before they went metric, one Swedish mile was 10,688 metres and a Norwegian mile was 11,295 metres, so only a small change had to be made to the old mile to make them equal to one myriametre. Even today, both Swedes and Norwegians use the 10,000 metre mile ("mil") to refer to travel distances in everyday language.
In other languages 
In the Sinosphere, which includes regional varieties of Chinese and East Asian languages that have used the Chinese writing system, words equivalent to Greek myriad "10,000" derive from the Chinese word wan (simplified Chinese: 万; traditional Chinese: 萬; pinyin: wàn; Wade–Giles: wan; literally "ten thousand"). Pronunciations of this Chinese character 萬 vary across linguistic areas. For instance, Cantonese maan6, Hakka wan5, Min Nan bān, Japanese man, Korean man (written as 만 in hangul), and Vietnamese vạn all mean "ten thousand; myriad".
The Chinese, Japanese, and Korean languages also have words for a myriad squared (100,000,000): yì (億) (or wànwàn [萬萬] in ancient texts), oku (億), and eok (억/億), respectively. A myriad cubed (10,0003 or 1012) is a zhào (兆), chō (兆), and jo (조/兆); a myriad to the fourth power (10,0004 or 1016) is a jīng (京), kei (京), and gyeong (경/京), a myriad to the fifth power (10,0005 or 1020) is a gai (垓), a myriad to the sixth power (10,0006 or 1024) is a shi (秭) in Korean, 자, a myriad to the seventh power (10,0007 or 1028) is a jō (穣)in Korean, 양 , a myriad to the eighth power (10,0008 or 1032) is a kō (溝) in Korean, 구, a myriad to the ninth power (10,0009 or 1036) is a kan (澗) in Korean, 간, a myriad to the tenth power (10,00010 or 1040) is a sē (正)in Korean, 정, respectively. Conversely, Chinese, Japanese, and Korean do not have single words for a thousand to the second, third, fifth power, etc., unlike English and many other European languages.
Other languages 
- Afrikaans: magdom
- Albanian: pafundësi
- Armenian: anhamar
- Azerbaijani: saysız-hesabsız
- Amharic: ′əlf (እልፍ)
- Hebrew: revava (רבבה)
- Biblical Hebrew: "ribo" (ריבוא)
- Malayalam: Ayutham (അയുതം)
- Tamil: Mudivili
- Telugu: Challa
- Thai: meun (หมื่น)
- Turkic: tümen
- Mongolian: Tüm (Түм)
- Tagalog: hindi mabilang
See also 
- Chinese numerals
- Indian numbering system
- Japanese numerals
- Korean numerals
- Myriad (typeface)
- Myriad (unit of area)
- Lakh (10 myriad)