Definition 1: An enormous host, a great throng (as a noun); countless, innumerable, existing in great numbers (as an adjective).

Usage 1: Today's word is a noun metamorphosing into an adjective. "Myriad" was originally a noun appearing in phrases like "a myriad of stars" but by the 19th century it was called to adjectival service, e.g. "myriad stars glowed above." Samuel Taylor Coleridge opened the 19th century with his famous phrase that demonstrated how "myriad" had assumed it new duty. In his 'Hymn to Earth' (1800), Coleridge wrote, "Myriad myriads of lives teem'd forth from the mighty embracement." Now that is a lot of lives.

Suggested usage: So, the story is that today's word originated as a noun: "Lionel is in possession of a myriad of excuses for not getting the job done." However, it works today just as well as an adjective: "He gave myriad reasons for firing me but not one that was cogent." yourDictionary predicts that by 2150 it will be an adjective pure and simple.

Etymology: Greek myrias, myriad- "ten thousand" from myrios "countless." (Myriad thanks to Joby Lewis for suggesting today's word with the ambivalent usage.)