I. ˈmir-ē-əd noun
Etymology: Greek myriad-, myrias, from myrioi countless, ten thousand
Date: 1555
1. : ten thousand
2. : a great number
a myriad of ideas
Usage:
Recent criticism of the use of myriad as a noun, both in the plural form myriads and in the phrase a myriad of , seems to reflect a mistaken belief that the word was originally and is still properly only an adjective. As the entries here show, however, the noun is in fact the older form, dating to the 16th century. 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.
II. adjective
Date: 1765
1. : innumerable
those myriad problems
also : both numerous and diverse
myriad topics
2. : having innumerable aspects or elements
the myriad activity of the new land — Meridel Le Sueur