Plural of millennium; periods of one thousand years, or informally, very long stretches of time.
From Latin 'millennium' (thousand years), from 'mille' (thousand) and 'annus' (year). The plural '-ia' is Latin, making it a classic Latinate plural like 'criteria' or 'phenomena.'
English speakers often say 'millennia' when they mean centuries—but 'millennia' means thousands of years, not hundreds, showing how even smart people mix up these epic time scales!
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