A period of ten years; a decade, especially as used in historical or technical contexts.
From Latin decennium, directly combining decem (ten) and annus (year). This is the original Latin noun form that English borrowed directly, and it appears in many formal historical and academic texts.
While 'decade' is what we usually say, 'decennium' shows up in ancient Roman histories because Latin scholars were actually counting these ten-year blocks for administrative purposes. It's like the original technical term before 'decade' became the everyday word!
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