decade (n.)

mid-15c., "ten parts" (of anything; originally in reference to the books of Livy), from Middle French décade (14c.), from Late Latin decadem (nominative decas), from Greek dekas (genitive dekados) "group of ten," from deka "ten" (from PIE root *dekm- "ten"). Meaning "period of ten years" is 1590s in English.

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