Plural form of decennium; multiple periods of ten years each.
The Latin plural of decennium, though sometimes English uses this form alongside the anglicized 'decades.' The -s plural ending is English's way of making Latin nouns fit into English grammar patterns.
Scholars love using decenniums when discussing multiple ten-year periods in ancient or medieval history—it sounds more formal and connects readers directly to how the Romans themselves thought about these time blocks. It's a word that makes you sound like you actually read primary sources!
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