myria-, myrio-, myri- (Greek: ten thousand; very numerous, countless; a number often used as a prefix).
This word has been used as a noun ever since its introduction into English in the mid-16th century. It was ultimately borrowed from a Greek word meaning "ten thousand", from a Greek adjective meaning "countless". In fact, from the 16th to the 19th century, the English noun was used to mean "ten thousand", primarily in translations from Greek or Latin, or in reference to the Greek numerical system.
In the modern sense, "a very great or indefinitely great number of persons or things", also dates back to the 1500's. The noun is used in both the singular (a myriad of voices) and in the plural (the myriads of voices; the voices of myriads). At the beginning of the 19th century, myriad began its life as an adjective meaning "countless, innumerable", but it was used mostly in poetry or in poetic prose.