A myriad (from Ancient Greek μυριάς, myrias) is technically the number 10,000 (ten thousand); in that sense, the term is used in English almost exclusively for literal translations from Greek, Latin or Sinospheric languages (Chinese, Japanese, Korean), or when talking about ancient Greek numerals.
In Classical Greek numerals, a myriad was written as a capital mu: Μ, as lower case letters did not exist in Ancient Greece. To distinguish this numeral from letters, it was sometimes given an overbar: M. Multiples were written above this sign, so that for example would equal 4,582×10,000 or 45,820,000. The etymology of the word myriad itself is uncertain: it has been variously connected to PIE *meu- ("damp") in reference to the waves of the sea and to Greek myrmex (μύρμηξ, "ant") in reference to their swarms.
The largest number named in Ancient Greek was the myriad myriad (written MM) or hundred million. In his Sand Reckoner, Archimedes of Syracuse used this quantity as the basis for a numeration system of large powers of ten, which he used to count grains of sand.
In English, myriad is most commonly used to mean "some large but unspecified number". It may be either an adjective or a noun: both "there are myriad people outside" and "there is a myriad of people outside" are in use. (There are small differences: the former could imply that it is a diverse group of people; the latter does not usually but could possibly indicate a group of exactly ten thousand.) The Merriam-Webster Dictionary notes that confusion over the use of myriad as a noun "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."
"Myriad" is also infrequently used in English as the specific number 10,000. Owing to the possible confusion with the generic meaning of "large quantity", however, this is generally restricted to translation of other languages like ancient Greek, Chinese, and Hindi where numbers may be grouped into sets of 10,000 (myriads). Such use permits the translator to remain closer to the original text and avoid repeated and unwieldy mentions of "tens of thousands": for example, "the original number of the crews supplied by the several nations I find to have been twenty-four myriads" and "What is the distance between one bridge and another? Twelve myriads of parasangs".
Most European languages include variations of "myriad" with similar meanings to the English word.
Additionally, the prefix myria- indicating multiplication times ten thousand (×104), was part of the original metric system adopted by France in 1795. Although it was not retained after the 11th CGPM conference in 1960, myriameter is sometimes still encountered as a translation of the Scandinavian mile (Swedish & Norwegian: mil) of 10 kilometers (6.21 mi), or in some classifications of wavelengths as the adjective myriametric. The myriagramme (10 kg) was a French approximation of the avoirdupois quartier of 25 pounds (11 kg) and the myriaton appears in Isaac Asimov's Foundation novel trilogy.
In Modern Greek, the word "myriad" is rarely used to denote 10,000, but a million is ekatommyrio (εκατομμύριο, lit. 'hundred myriad') and a thousand million is disekatommyrio (δισεκατομμύριο, lit. 'twice hundred myriad').
In East Asia, the traditional numeral systems of China, Korea, and Japan are all decimal-based but grouped into ten thousands rather than thousands. The character for myriad is 萬 in traditional script and 万 in simplified form in both mainland China and Japan. The pronunciation varies within China and abroad: wàn (Mandarin), wan5 (Hakka), bān (Minnan), maan6 (Cantonese), man (Japanese and Korean), and vạn (Vietnamese).
Because of this grouping into fours, higher orders of numbers are provided by the powers of 10,000 rather than 1,000: In China, 10,0002 was 萬萬 in ancient texts but is now called 億 and sometimes written as 1,0000,0000; 10,0003 is 1,0000,0000,0000 or 兆; 10,0004 is 1,0000,0000,0000,0000 or 京; and so on. Conversely, Chinese, Japanese, and Korean generally do not have native words for powers of one thousand: what is called "one million" in English is "100萬" (100 myriad) in the Sinosphere, and "one billion" in English is "十億" (ten myllion) or "十萬萬" (ten myriad myriad) in the Sinosphere. Unusually, Vietnam employs its former translation of 兆, một triệu, to mean 1,000,000 rather than the Chinese figure. Similarly, the PRC government has adapted the word 兆 to mean the scientific prefix mega-, but transliterations are used instead for giga-, tera-, and other larger prefixes. This has caused confusion in areas closely related to the PRC such as Hong Kong and Macau, where 兆 is still largely used to mean 10,0003.
萬 and 万 are also frequently employed colloquially in expressions, clichés, and chengyu (idioms) in the senses of "vast", "numerous", "numberless", and "infinite". A skeleton key is a 万能钥匙 ("myriad-use key"), the emperor was the "lord of myriad chariots" (萬乘之主), the Great Wall is called 万里长城 ("Myriad-mile Long Wall"), Zhu Xi's statement 月映万川 ("the moon reflects in myriad rivers") had the sense of supporting greater empiricism in Chinese philosophy, and Ha Qiongwen's popular 1959 propaganda poster 毛主席万岁, meaning "Long live Chairman Mao", literally reads as "[May] Chairman Mao [live to be] 10,000 years old".
A similar term is the Old Turkic word tümän, whose variant forms remain in use for "ten thousand" among modern Mongolian, Turkish. According to Sir Gerard Clauson (1891–1974), it was likely borrowed from Tokharian, which may have been borrowed in turn from Old Chinese.