Myriad (μυριάδος, from μύριος, murios, "numberless, countless, infinite", later converted to "myriad" by the Romans) is a classical Greek name for the number 10,000. In modern English, the word refers to an unspecified large quantity.
The term myriad is a progression in the commonly used system of describing numbers using tens and hundreds. Small numbers are named in terms of number of tens plus the remainder; for example 76 is seven tens plus six. Numbers larger than ten tens require a new description, a hundred. Thus, 1776 is seventeen hundred seventy six. Similarly one hundred hundred is a myriad. A myriad myriads, or one hundred million, was left as the largest named number by the Ancient Greeks and is also the largest named number in the Bible.
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 a noun or an adjective. 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. There is no reason to avoid it."
The English numbering system divides large numbers into groups of three digits, and so the names for such numbers follow this division (10,000 = ten thousand). East Asian numbering divides large numbers into groups of four; so in Chinese, Japanese, or Korean, 30,000 really would be "three myriad" (3,0000 — Japanese san-man). One million is a hundred myriad (100 × 10000 instead of 1000 × 1000); the next uniquely named number after a myriad is 億, which is myriad myriad (10000 × 10000) or a hundred million.
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, Swedes normally use the Swedish mile to refer to travel distances in everyday language. Similarly, one myriometre = 0.0001 metre.
Chinese, Japanese and Korean 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)(pronounced "uhk"), respectively. A myriad cubed (10,000³ or 1012) is a zhào (兆); chō (兆); a myriad to the fourth power (10,0004 or 1016) is a jīng (京); kei (京). Conversely, Chinese, Japanese and Korean do not have single words for a thousand squared, cubed, etc., unlike English and many other languages of European origin.