What is myriad?

myrÂ·iÂ·ad (mr-d)

ADJECTIVE:
Constituting a very large, indefinite number; innumerable: the myriad fish in the ocean.

Composed of numerous diverse elements or facets: the myriad life of the metropolis.

NOUN:
A vast number: the myriads of bees in the hive.

Archaic Ten thousand.

ETYMOLOGY:
Greek mrias , mriad-, ten thousand, from mrios, countless

Usage Note:
Throughout most of its history in English myriad was used as a noun, as in a myriad of men. In the 19th century it began to be used in poetry as an adjective, as in myriad men. Both usages in English are acceptable, as in Samuel Taylor Coleridge's "Myriad myriads of lives." This poetic, adjectival use became so well entrenched generally that many people came to consider it as the only correct use. In fact, both uses in English are parallel with those of the original ancient Greek. The Greek word mrias, from which myriad derives, could be used as either a noun or an adjective, but the noun mrias was used in general prose and in mathematics while the adjective mrias was used only in poetry.

Individuals are just copying other people's erroneous usage. What I want to know is who started this mess?
Actually, myriad has been used as a noun ever since its introduction into English in the mid-16th century. It was borrowed ultimately 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', chiefly in translations from Greek or Latin, or in reference to the Greek numeral system: "Rome...vaunted herself of two myriades of inhabitants." (Burton, The Anatomy of Melancholy, 1621) But the modern sense, 'a very great or indefinitely great number of persons or things', also dates back to the 1500s. The noun is used in both the singular (a myriad of voices) and 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 poetic prose: "Myriad myriads of lives teem'd forth from the mighty embracement" (Coleridge, Hymn to Earth) "Thou of the many tongues, the myriad eyes!" (Tennyson, Ode to Memory) "It describes myriad circles in the sand...a myriad little cables of beach-grass." (Thoreau, Cape Cod) The adjective is also used with a singular noun to mean 'having innumerable phases, aspects, or variations', as in the example "the myriad mind of Shakespeare."

The use of the adjective in poetic language became so common that people began to think the noun was incorrect. You are not the first person who has written to us about this "erroneous" use of myriad. But to answer your question, all modern usage guides consider the noun to be acceptable.