An extremely large, indefinite number; originally meaning exactly ten thousand.
From Greek myrios meaning countless or ten thousand. In ancient Greece, myriad was the largest named number, equivalent to 10,000, but evolved to mean simply 'countless'.
The shift from a specific number (10,000) to meaning 'countless' shows how language inflates over time. Ancient Greeks would be amused that their word for their biggest number now means any really large amount.
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