I was reading a New York Times article about Obama's budget when I saw this sentence: "Mr. Obama aims to move workers into the first category (of college graduates) by increasing federal financial aid and simplifying the myriad of aid programs." I thought, "A-ha! An error I can write about." I thought that myriad is properly an adjective meaning "countless" and not a noun meaning "a great number," and that it should be used this way: "simplifying the myriad aid programs."
But when I went to start looking for supporting evidence, I discovered that this was another "rule that isn't."
Garner's Modern English Usage says that while "myriad is more concise as an adjective ... the mere fact that the adjective is handier than the noun doesn't mean the latter is substandard. The noun (ca. 1555) has been with us more than 200 years longer than the adjective (ca. 1791), and the choice is a question of style, not correctness."
Other reference books agree. Merriam Webster Online: "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."
In the 19th century (myriad) 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 murias, from which myriad derives, could be used as either a noun or an adjective, but the noun murias was used in general prose and in mathematics while the adjective murias was used only in poetry.
The Greek word murias referred to above is ancient Greek for 10,000. According to the OED, it also had the non-literal meaning of "countless" in ancient Greek, but that was "poetic and rare." Myriad came into English from the post-classical Latin myriades (plural) also meaning "multiples of ten thousand, a countless number."