A vast indefinite number.
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 mūrias, from which myriad derives, could be used as either a noun or an adjective, but the noun mūrias was used in general prose and in mathematics while the adjective mūrias was used only in poetry.