Decade

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late Middle English (denoting each of ten parts of a literary work): via Old French and late Latin from Greek deka ‘ten’. decade (sense 1) dates from the early 17th century.

wiktionary

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From Middle English[Term?], from Middle French decade, from Late Latin decas(“(set of) ten”), from Ancient Greek δεκάς(dekás), from δέκα(déka, “ten”). In reference to a span of ten years, originally a clipping of the phrase decade of years. The word is equivalent to deca- +‎ -ade.

etymonline

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decade (n.)

mid-15c., "ten parts" (of anything; originally in reference to the divisions of Livy's history), from Old 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 consecutive years" is 1590s in English. Related: Decadal; decadary.