Myriad, which means innumerable, is one of those live-and-learn words for me. The impetus for writing about it was my recent reading it in several books in its noun form, that is, “a myriad of,” rather than its adjectival form. I was all set to go through a lengthy explanation of how myriad was being abused by those nouners only to discover that it really does belong on my live-and-learn list.
I was always taught that myriad is an adjective and should be used thusly, “Jones had myriad questions about the new water heater” and never “Jones had a myriad of questions about the new water heater.” I even remember in the early days of my editing career having this pointed out to me by both a client and fellow editors who I had queried on the subject. Turns out they were all wrong!
Myriad began life in the mid-15th century as myriade (Middle French) meaning 10,000, or perhaps has roots in the Greek murias or muriad-, from muriot or 10,000, passing on to the Middle French via the Late Latin myrias. According to Bryan Garner (Garner’s Modern American Usage ), the noun form has been with us since circa 1555 (Chambers Dictionary of Etymology  notes that the figurative sense of countless numbers was first recorded in 1555), whereas the adjectival form didn’t come on the scene until circa 1791. The American Heritage Guide to Contemporary Usage and Style (2005) attributes the birth of the adjectival form to its use in poetry in the early 18th century. Coleridge’s poem Hymn to Earth is a noteworthy example of its adjectival use in 18th century poetry:
Mightier far was the joy of thy sudden resilience; and forthwith
Myriad myriads of lives teem’d forth from the mighty embracement.
So I’ve learned that myriad can be both noun and adjective. Clearly, its usage now is a matter of style rather than correctness vs. incorrectness.