Q: Some people use the word “myriad” as an adjective meaning numerous, others as a noun meaning a large number, and still others as a noun meaning 10,000. Which is correct?
A: “Myriad” is both a noun meaning a great number and an adjective meaning numerous. In fact, the noun came first (circa 1555, vs. 1735 for the adjective). Although “myriad” is derived from two Greek words – one meaning 10,000 and the other innumerable – the sense of 10,000 is archaic today.
I agree with Bryan A. Garner, who notes in Garner’s Modern American Usage that the word “is more concise as an adjective (myriad drugs) than as a noun (a myriad of drugs).” But Garner adds that the choice between noun and adjective “is a question of style, not correctness.”
In my opinion, “myriad,” in the sense of an indefinite large number (which is the modern meaning), is redundant in the plural (“myriads”). I’ve maintained this view in both the first and second editions of my book Woe Is I.
I’m well aware that the plural “myriads” is a very old usage (in fact, it reeks of antiquity). But since the noun “myriad” now means a great number, the plural construction (“myriads”) seems unnecessary. And while we’re on the subject of style, not correctness, I regard “a myriad of” as infelicitous for much the same reason.
But it would be wrong to call those usages incorrect. In fact, they’re “parallel with those of the original ancient Greek,” according to a usage note in The American Heritage Dictionary of the English Language (4th ed.).
[Update: The third edition of Woe Is I (2009, 2010) puts petty prejudice aside and bows to current usage, infelicitous though it may be. Here’s the updated entry: “MYRIAD. It originally meant ‘ten thousand,’ but myriad now is an adjective meaning ‘numerous’ (Little Chuckie has myriad freckles) or a noun meaning ‘great number’ (He has myriads [or a myriad] of them).”]
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