Plural of decennium; periods of ten years or decades.
From Latin decennium, composed of decem (ten) and annus (year). The term evolved in English to refer to ten-year periods, with the plural form deccennia maintaining Latin grammatical structure.
This word is a fossil from when English scholars insisted on using Latin plurals for technical terms—today we'd just say 'decades,' but deccennia survives in specialized historical and scientific writing, showing how some languages refuse to die.
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